Quality Market: Design and Field Study of Prediction Market for Software Quality Control
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Proceedings of the 44th Hawaii International Conference on System Sciences - 2011 Quality Market: Design and Field Study of Prediction Market for Software Quality Control Abstract the software industry and the critical consequences of Given the increasing competition in the software software errors, it has become important for industry and the critical consequences of software companies to achieve high levels of software quality. errors, it has become important for companies to Project managers will benefit greatly if forecast on achieve high levels of software quality. Generating confidence in software quality is available early in early forecasts of potential quality problems can have development cycle. significant benefits to quality improvement. In our research, we utilized a novel approach, There are various ways to define software quality called prediction markets, for generating early and since quality is a multi-faceted concept, it is best forecasts of confidence in software quality for an understood from a well-defined perspective. For the ongoing project in a firm. Analogous to financial purpose of this research, we take a holistic view of market, in a quality market, a security was defined software product quality as one that combines the that represented the quality requirement to be views of the users, quality assurance members, predicted. Participants traded on the security to quality managers along with the developers and the provide their predictions. The market equilibrium management team. Being able to measure quality price represented the probability of occurrence of the early and as needed enables the use of early forecast quality being measured. The results suggest that to take corrective actions. Thus, a software quality forecasts generated using the prediction markets are estimation mechanism should i) provide estimation closer to the actual project outcomes than polls. We early in development cycle, and ii) take into account suggest that a suitably designed prediction market quality input from multiple stakeholders. may have a useful role in software development domain. One such mechanism is called a prediction market (PM, henceforth). A prediction market is analogous to a stock market (specifically, futures markets). 1. Introduction Theory and empirical evidence suggest that prediction markets work very well in aggregating Among many practical challenges in software opinions from diverse stakeholders across many engineering is the estimation task – the estimation of domains. Prediction markets are also easy to set up cost, timeline, delivery date, and software quality or and administer. assurance. According to National Information Assurance Glossary, Software Assurance is defined The purpose of this research is to evaluate as “the level of confidence that software is free from whether a prediction market for software quality can vulnerabilities, either intentionally designed into the be used to forecast quality problems early in the software or accidentally inserted at anytime during its project. lifecycle”. To that end, software assurance encompasses the development and implementation of 2. Background and Research Questions methods and processes for ensuring that software functions as intended while mitigating the risks of 2.1 Software Quality vulnerabilities, malicious code or defects that could bring harm to the end user. One such process is the The IEEE standard (1061-1992) for software testing and verification process. This process verifies quality metrics methodology recommends that a and validates coding during each stage of the de- software implementation project should develop a velopment process. It ensures that the concept is methodology for establishing quality requirements complete and that all requirements are well- and a process for validating the quality metrics. One implemented and function as intended. While cost such process described in the standard is called reduction and timeliness of projects continue to be Predictive Metric, which provides advice on important measures, software companies are placing identifying a metric to be used during the increasing attention on identifying the user needs and development phase to predict the eventual values of a better defining software quality from a customer software quality factor. perspective [14]. Given the increasing competition in 1530-1605/11 $26.00 © 2011 IEEE 1
Proceedings of the 44th Hawaii International Conference on System Sciences - 2011 In a traditional software estimation process, the parameters of interest defined by the market designer. managers along with the developers arrive at the For example, a contract can be defined on the number estimation figures. The estimation process does not of defects likely to be observed at a particular stage include individuals from business domain, testers or in the software development process. A simple project sponsors. Research in group dynamics has contract could specify the price for the contract when demonstrated that, in general, the consensus of a the number of defects is less than an integer K is p. group is better than any one individual’s judgment Traders have some private information about the (popularized as "wisdom of crowds" by Surowiecki) defect rate and can observe the current market price [15]. p. If a trader believes that the contract is underpriced (i.e., there would be fewer defects than p would 2.2 Prediction Markets indicate), then she can purchase the contract so as to maximize her returns. Likewise, a trader will sell a A prediction market (PM) is similar to a stock contract if she believes it is overpriced. The process exchange and well-designed prediction markets for of buying and selling thus, reveals information held forecasting purposes have been developed for a by traders. When the price reaches an equilibrium variety of situations. The Iowa electronic markets, level, the no trader has an incentive to buy or sell, conducted by University of Iowa, are used to predict given her private information and the market is political outcomes are among the best known of closed. The equilibrium price, thus, reflects aggregate prediction markets in operation. Apart from political information available among the traders. markets, Prediction markets have been used to forecast movie revenues, corporate sales, project 2.4 Research Questions completion, and economic indicators [17]. In this research, we use a suitably designed Considerable theoretical and empirical support prediction market for forecasting a particular attribute exists for the superior performance of well-designed of software - called software correctness. For markets to forecast future outcomes. Wolfers and comparative purposes, we evaluate the forecasts Zitzewitz [17, 18] analyzed the extent to which generated by a PM against those generated by a prediction markets can be used to aggregate disperse simple poll and the actual outcomes available at information into efficient forecasts of uncertain project completion. future events. Drawing together data from a range of prediction contexts, they show that market-generated This research used a field study approach and forecasts are typically fairly accurate, and that they stakeholders in a live project serve as participants. outperform most moderately sophisticated The purpose of the study was to explore the benchmarks. effectiveness of prediction markets in forecasting software quality factors. The two research questions 2.3 Prediction Markets for forecasting addressed in this research are: software quality 1. How well does a prediction market forecast Prediction markets can be used to forecast many software correctness compared to opinion aspects of the software project - in this research, we polls? focus on quality. A prediction market, because it is 2. How well does a prediction market forecast easy to set up and conduct, can be used at any stage software correctness compared to actual of the software development project. Second, it is measures of software correctness? rather straight forward to include different stakeholders in the market. Since PM's are known 3. Market Design (theoretically as well as empirically) to aggregate information from multiple decision makers 3.1 Experiment efficiently, a PM can yield a much better forecast than similar methods. Further, since trading in a The experiment was conducted in a major Wall prediction market can be made anonymous, it Street financial institution in Northeast America. encourages employees to share unwelcome With the consultation of the project management information about a project’s launch date or team, an on-going software development project was performance without fears. chosen for this study. The project was a small size project to support securities trading at the firm. In a prediction market, various stakeholders Members of the project, including one sponsor, one (called traders) buy or sell contracts on some 2
Proceedings of the 44th Hawaii International Conference on System Sciences - 2011 project manager, one technical manager, three developers, one tester, two users and a development Three different incentive structures were team lead participated in the study. An online virtual considered for this study: stock market was developed for this experiment and 1. a constant amount to be paid to all participants made available for participants to trade. The market 2. participants’ reward can be linearly dependent on was hosted on a public domain and was made the final net worth and all participants will be available 24x7. In this experiment the participants paid at the end of the experiment, or played the role of traders buying and selling shares of 3. the top winner can get $300, the 2nd top winner the contract with virtual currency (or play money). $200 and the 3rd winner can get $100; The shares themselves carried no value as they were traded with fictitious money. Since they had no value Since these options involve real money reward, of their own, they were used to induce values through there might be legal and technical difficulties an appropriate reward mechanism [13]. involved in actually implementing the incentive structure. Thus, we asked the subjects to trade so as 3.2 Contract to maximize their final net worth in play money. Subjects with the highest net worth in play money at In this experiment the event in question that the end of the market session will be awarded an needed to be forecasted was the software correctness. extra vacation day by the manager and others would Software correctness is defined as the extent to which not get any incentive. software satisfies its specifications and fulfills the users’ tolerance limits. The contract in this case, called SC_contract, was defined as below: 3.5 Instructions to Subjects SC_Contract: What percentage of specifications will The following instructions were provided to the the final software fulfill? subjects prior to the experiment. i. The participants should not share their userid 3.3 Trading Platform and password with other participants, nor participant in trade with others subject's login Participants used a web-based prediction market id's. to trade contracts representing the two outcomes. A ii. It was suggested that all requirements of the subsidizing market-maker based on a Hanson’s software project be considered to be of equal logarithmic scoring rule was used to ensure liquidity weight. No special weights are given based despite the small number of traders and two outcome on priority/complexity of the requirement. space [4]. After an initial instruction period on a practice market, each participant received login iii. If a requirement is partially implemented or details for a trading account that was funded with fully not functional, then the requirement is 100,000 play money units. The initial price of the considered not implemented for the contract was set at 0.80. The market was open 24x7 percentage calculation. during each stage. Initial test run was conducted at the project site for a week for learning and any We believe that subjects did adhere to the improvements to the market design. instructions during the market sessions and outside. 3.4 Participant Incentives 3.6. Experimental Sessions Incentives are usually a matter of serious debate Subjects judged the probability of meeting among experimental researchers. In experimental requirements using the prediction market (PM). A economics literature, Smith [13] suggests that using second method of indicating the response was by monetary rewards increases the salience of the task supplying a probability number at the end of a trading and shows that inexperienced subjects converge session and is termed as a Poll. Under the Poll toward “rational” behavior more rapidly as the size of treatment, subjects do not have an opportunity to rewards are increase. In general, psychologists do not revise their estimates - thus, data obtained through emphasize incentives as much as economists do. In polls can be considered "naive" judgments while data the context of online prediction markets, Wolfers obtained from the PM can be considered informed et.al. [19] find that usage of play versus real money judgments. Finally, data on actual progress of the did not make a difference to the forecast quality. project was collected and this serves as the actual or 3
Proceedings of the 44th Hawaii International Conference on System Sciences - 2011 objective data that PM and Poll were trying to The following table provides a preliminary forecast. summary of the results. Data were collected at three different stages in a Table 2. Preliminary Summary of Results live, ongoing software project at a client's location PM Stages Closing Mean of Project during the prediction market sessions. The three Bid for Poll Actual separate stages are: Requirements, Release1, and PM Forecasts* Release2 (Final Implementation). Requirements 0.97 0.91 0.76 Release1 0.78 0.67 0.76 Ten subjects participated in both the PM and the Release2 0.75 0.69 0.76 Poll treatments. In the case of Poll treatment, each *Mean of poll estimates from 10 participants subject provided a probability at the end of the stage thus yielding 10 observations for analysis. In the case Data in Table 2 suggests that, while the of the PM treatment, a subject could provide multiple requirements stage data for the closing bid for the PM estimates until the market for that stage was closed. is quite different from actual error rate, Release1 and Thus, the number of predictions or observations can Release2 data is rather close. The data from poll be larger than 10 even though the number of subjects means is quite far apart from actual project data and is still ten. The Table 1 below summarizes the is a less accurate predictor of the actual data experiment. compared to the PM for Release1 and Release2 stages. While it is tempting to conduct statistical Table 1.Experiment Design significance tests using Poll data, given the numerous Treatments issues with the sample size and distribution, we do Stages Prediction Poll not report the results of a test. Detailed analysis is Market presented below. Requirements Number of Number of subjects = 10, subjects= 4.2 Data Characteristics Number of Number of predictions = 20 predictions = 10 Data collected through this experiment has Release 1 Number of Number of several characteristics which are common to field subjects = 10, subjects= experiments run with a live software project. First, Number of Number of the number of subjects who participated in the predictions = 39 predictions = 10 software project is small - ten to be exact. Second, Release 2 Number of Number of the same subjects provide PM and Poll treatments subjects = 10, subjects= (i.e., within subject design) first by participating in Number of Number of PM and then providing Poll data (i.e., without predictions = 29 predictions = 10 counterbalancing). Third, subjects in PM treatment provide multiple revised estimates which are likely to 4. Analysis be correlated. Fourth, the distribution of estimates among subjects is not unimodal (discussed below). Thus, it is unlikely that any statistical test would have 4.1 Preliminary Analysis sufficient power if used for testing statistical significance. Two specific hypotheses, derived from the research questions are stated below. The first Thus, in the following analysis, we report the hypothesis compares the forecasts between the PM complete distribution of the data obtained from the and Poll treatments and is stated as follows: experiment. This makes sense to us given the H1: The PM forecast is not significantly different relatively low power of any test with such sample from Poll forecast. sizes. A stronger test is the comparison between PM forecasts and the actual, objective project outcomes. 4.3 Further Analysis for H1 The hypothesis can be stated as: For the PM case, ten subjects provided a total of H2: The PM forecast is not significantly different twenty bids or predictions. The number of from the actual project outcome. predictions exceeds the number of subjects because each subject is allowed to bid as many times as 4
Proceedings of the 44th Hawaii International Conference on System Sciences - 2011 needed until the end of the PM session. All subjects insufficient information about the software project at were made aware of the ending time of the PM this time for making informative judgments as well session. as to revise beliefs. 4.3.1 Requirements Stage analysis for H1: 4.3.2 Release 1 stage analysis for H1 Data collected at the end of requirements stage for After the requirements stage, the software team poll and PM treatments was subjected to a non worked on the project for three weeks and released an parametric test (Mann-Whitney). The PM treatment early version of the product. We call this Release 1 has N=20 predictions (each subject, on average, and discuss data collected after this stage through the revised his estimate once) and the PM treatment has a PM and Poll. The subjects knew about what the mean of 90.85 and a standard deviation of 4.36. features are being released via a central repository Immediately after the PM was closed, subjects database maintained at the firm. participated in a poll (ten predictions, one per subject) which has a mean of 90.6, and a standard Subjects provided one estimate each for deviation of 6.19. A Mann-Whitney test, based on probability judgment of contract completion and the median ranks, yields a one-sided (PM > poll) p-value Poll line shows the distribution. The same subjects = 0.482 and two-sided (PM poll) = 0.965. Thus, it revised their estimates multiple times in the PM is concluded that there is no significant difference session (39 estimates of probability judgment by 10 between PM and Poll data. Thus, the null hypothesis subjects) and the data from all 39 judgments is of no difference between PM and Poll is supported. presented as PMAll. The last prediction from each subject, prior to market close is presented as PMclose More insight is obtained by viewing the data (thus, this line plots 10 observations). The data is distribution presented in Fig. 1 below. The x-axis presented in Fig.2 and we discuss the data refers to the forecast and the y-axis to the frequency distribution intuitively rather than rely on a statistical of the forecast (normalized by dividing with the test of questionable power. number of bids, so that they can fit into the same graph). In Fig.1, we represent the distribution of 1. The Poll data has a clear mode at about 65% forecast data using requirements stage data. Two and is tightly dispersed at the mode. versions of PM data are presented - PMAll denotes 2. The PMAll data contains all the data all predictions made by subjects during the including revised beliefs. experiment and thus reflects multiple revised 3. The PM Close distribution is nearly uniform forecasts by subjects while PMClose denotes the last with support between [60%, 80%] and is prediction (one for each subject) before the PM was significantly different from Poll data. closed. Thus, while Poll and PMClose have 10 observations, PMAll can have more than 10 We interpret the data as suggesting that PM observations. and Poll yield different forecasts at the Release 1 stage. We can see that PMAll data has a bi-modal distribution with one mode near 85% and another at 4.3.3 Release 2 stage analysis for H1 95% while the poll data seems have one clear mode The software was worked on further and a at 85%. PM Close line shows the distribution of data, different and final version was released as Release 2. one per each subject, prior to market closing - thus Fig. 3 contains the distribution of forecasts obtained the mean of PM close is the equilibrium price. thru Poll and PM methods. 1. The Poll shows two modes with a prominent Note that the Poll mode (at 85%) nearly coincides mode at 70. with the PMAll mode (at 85%) - thus, it can be 2. The PMAll data, because it has numerous argued that subjects started with an estimate of 85% modes, is nearly un-interpretable. The PMClose chance that the contract of >80% specifications data is dispersed narrowly with support in [72%, fulfilled. However, after participating in the PM and 80%] range with a prominent mode at 75%. observing other people's bids, a majority seems to have changed their judgments and the mode in PM A Mann-Whitney test for median differences close suggests that most subjects believed that the between PMClose and Poll indicates a statistically probability of meeting the contract is around 95%. significant difference. Visually scanning the two distributions also suggests that PMClose distribution We feel that since this data was collected at the is different from the Poll forecast distribution. early requirements stage, there is probably 5
Proceedings of the 44th Hawaii International Conference on System Sciences - 2011 Overall, we conclude that PMClose Release 2 Yes forecasts are different from the forecasts generated through a Poll at Release 2 stage. 4.4 Further Analysis for H2 Req. Prob. Distribution The PM and poll are two different ways of 0.3 forecasting probabilities. The key question, however, 0.27 0.24 is whether one or the other method is a good Probability 0.21 0.18 predictor of eventual success rate for the software 0.15 0.12 PM project. The following analysis focuses on the second 0.09 0.06 0.03 Poll question which is repeated below: 0 PM Close 70 75 80 85 90 95 100 How well does a prediction market forecast software Estimates correctness compared to actual measures of software correctness? Fig1. Probability Distribution for H1 with Req. data The actual error rate in the software project used in the task was assessed by the project manager on completion of the project (i.e., after Release 2 stage) Rel1. Prob. Distributions to be 76%. This was arrived by manually counting 0.6 0.56 the number of specifications that were fully 0.52 0.48 0.44 functional. The number of original specifications for 0.4 implementation was 25 and after the Release2, the Probability 0.36 0.32 PM All 0.28 0.24 0.2 project manager counted the user approved Poll 0.16 0.12 0.08 specifications that were fully functional which turned PM close 0.04 0 out to be 19 that makes the actual error rate to be 45 50 55 60 65 70 75 80 85 90 95 100 76%. Estimates The hypotheses can be stated as follows: H2: At [requirements/release 1/ release 2] stage, the Fig2.Probability Distribution for H1 with Rel1 forecast using [PM/poll] is the same as actual error data rate of 76%. H2a: The forecasts are different from true error rates. Rel2. Prob. Distribution The data is summarized below in Figures 4-6. 0.4 Note that the data for PMAll, PMClose and Poll is 0.36 0.32 identical to those in the first set of graphs (Fig. 1-3). 0.28 The actual error rate is overlaid on the same graphs Probability 0.24 0.2 PM All as a visual guide. Due to issues of small sample size, 0.16 0.12 Poll multimodality of distributions and correlation among 0.08 0.04 PM close forecasts, we chose not to use statistical tests for 0 significance. Instead, we interpret the data based on 50 55 60 65 70 75 80 85 90 95 100 the distributions and note that our conclusions may Estimates not be statistically significant and other interpretations are possible. Fig3.Probability Distribution for H1 using Rel2 data In summary, the results are: Table 3: Forecasts from PM and Poll at stages Stage Is PM different from poll? Requirements No Release 1 Yes 6
Proceedings of the 44th Hawaii International Conference on System Sciences - 2011 1 are indistinguishable from true error rates while Poll forecasts fall short. Figure 6 contains the data for Release 2 stage. Poll data has much of distribution to the left of the true error rate and consistently underestimates it. The forecasts obtained from subjects prior to market close, or PMClose, have two modes on either side of the true rate of data and narrower support of [75,80%] around the true rate of 76%. Thus, Poll forecasts seem different from actual while PMClose data do not. Fig 4. Probability distribution for H2 at Requirements The results of our analysis are summarized in the table below: Table 4: Summary of Analysis Stage Is the PM Is the poll forecast forecast different from different Actual? from Actual? Requirements Yes Yes Release1 No Yes Fig 5. Probability distribution for H2 at Release1 Release2 No Yes 5. Summary, Limitations and Future Research 5.1 Summary In this research, we use a prediction market to generate aggregate forecasts of quality judgments for a software project in progress. Ten stakeholders Fig 6. Probability distribution for H2 at Release2 including business managers, project management team, development team and end user community are Figure 4 presents Poll data (mean forecast of 91%) used as subjects. The ten subjects provide their and PMClose data (mean forecast of 92%) as well as forecasts at three different stages of the project - at actual error rate (76%) for the requirements stage. requirements stage, at an early release stage and a We judge the situation as one in which neither the final release stage. Subject judgments of an aspect of Poll method nor the PM method as being good at quality (specification completeness) is assessed using forecasting the true error rate. the PM and Poll (a "naive" bench mark) at the three stages. On completion of the project, the true error Figure 5 contains the data for Release 1 stage. The rate in the project is collected as well. Poll has a unimodal distribution with the mode at 65% and all data fall within [60%, 75%]. Thus, Poll An analysis of data suggests that, as one data at Release 1 stage does not seem to predict true progresses through the stages of software error rates correctly and definitely underestimates it. development from requirements to later releases, the The PMClose distribution is nearly uniform with differences in predictions from PM diverge from support between [60%, 85%] with a mean around those in a Poll. Unlike in a Poll, in a PM subjects can 73%. We thus conclude that PM forecasts at Release use the market information available thru ongoing 7
Proceedings of the 44th Hawaii International Conference on System Sciences - 2011 trades on the contract and thus adjust their software releases for next year (particularly during predictions. holiday season). These predictions could help the management in aligning the resources appropriately. Comparison of PM and Poll forecasts with the true outcomes suggests that forecasts generated by subjects when using PM are closer to the true error 6. References rates than forecasts generated thru Polls. Thus, this study provides preliminary evidence to using the PM [1] Briand, L.C., Basili, V.R. and Hetmanski, C.” method for predicting software forecasts. Developing interpretable models for optimized set reduction for identifying high-risk software components,” 5.2. Limitations of the Study IEEE Transactions on Software Engineering, 1993, pp 1028–1034. The application of PM to software project [2] Cavano, J., McCall, J. “A framework for the milestones is new and conducting one using a live measurement of software quality”, Proceedings of the project in the field (as opposed to the lab) placed software quality assurance workshop on Functional and considerable constraints on our ability to control the performance issues 1978, pp 133-139. environment. Since this is a novel application, we had to settle for a small scale project. Ideally, a [3] Grosser, D., Sahraoui, H.A. and Valtchev, P. “Analogy- prediction market can be "designed" for each based software quality prediction.” Object-Oriented forecasting task. In this study, we did not have the Software Engineering, 2003. luxury of "designing" a mechanism. [4] Hanson, R. and Oprea, R.” Manipulators Increase This was the first time that the Wall Street Information Market Accuracy”, 2005, George Mason Company employed a virtual market for software University. estimation and the participants were especially [5] ISO/IEC 9001:2000. Quality management systems— delighted about using the market. To some extent, Requirements, International Organization for this mitigated the weaker incentive system (one Standardization. vacation day to the winner in the trading) because we felt that the subjects were quite motivated. [6] Juran J. and Gryna F. Quality Planning and Analysis, 2nd ed., McGraw-Hill, New-York., 1980. 5.3 Suggestions for Future Work [7] Khosgoftaar, T.M and Munson, J.C. “Predicting In this study, the forecasts of the PM are software development errors using software complexity compared with a Poll and actual outcomes. Polls may metrics.”, IEEE Journal on Selected areas in Communications, 1990. be viewed as a "naive judgment aggregation" mechanism and future research might use alternate mechanisms other than Polls as a baseline in testing [8] Khosgoftaar, T.M., Lanning, D.L., and Pandya, A.S.” A comparative study of pattern recognition techniques for PM's. quality evaluation of telecommunications software,”, IEEE Journal on Selected areas in Communications, 1994, As a future study, a suggested use of PM could be pp 279–291. to consider the market concept as a means to estimate the confidence in quality estimates. That is, as a [9] Li, P.L, Herbsleb, J., Shaw, M., and Robinson, B. secondary perspective or validation rather than the “Experiences and results from initiating field defect primary estimate. prediction and product test prioritization efforts at abb inc.”, Proceedings of The 28th International Conference on In this study, we used a specific attribute of Software Engineering, 2006. quality called software correctness as the object of forecast. Future research could also consider using [10] Nagappan, N., Williams, L., Vouk, M., and Osborne, contracts on multiple attributes such as a joint J. “Early estimation of software quality using in-process prediction task in which both correctness and say, testing metrics: a controlled case study,” Proceedings of the usability are traded in a PM. PMs could also be used third workshop on Software quality, 2005, pp 1-7. in other project management tasks such as predicting [11] Paulk, M.C., Weber, C.V., Curtis, B., and Chrissis, implementation date and project cost. In addition M.B. The Capability Maturity Model: Guidelines for PMs can be used in organizational management Improvement of the Software Process, Addison-Wesley., decisions such as software product sales, number of 1995. 8
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