22 Several weeks later, each student developed a few sce-narios for the initial issues and used them as the basisfor a new round of best guesses and confidence ranges(ignoring the first round). As before, each subject re-quested new estimates from the colleague at work afterexplaining the specific scenarios. This way I could testwhether the scenarios had any systematic effect on theestimates of either the students or their colleagues.Averaged across all cases, confidence ranges widenedabout 50 percent. The scenarios affected best guessesless than ranges, although there was considerable vari-37SLOANMANAGEMENTREVIEW/WINTER1995SCHOEMAKERTable 5Positive versus Negative Elements in ScenariosMeanStandard DeviationSample SizeSignificanceNumber of positive trends identified per subject2.00.8359.001Number of negative trends identified per subject1.48.9459Number of positive outcomes listed per subject3.58.8859.16Number of negative outcomes listed per subject3.42.8159Subjective probability of positive outcomes51%1256.001Subjective probability of negative outcomes43%1356Plausibility of the positive scenario (1-7 scale)22.214.171.124Plausibility of the negative scenario (1-7 scale)4.51.256Note: Significance levels in the last column refer to a t-test for differences in means between positive and negative items within each pair (with a z-test for the third pair). Each subject identified about four trends on average (including indeterminate ones in terms of impact) and identified 3.7 uncertainties on average.
ability in both. Because the scenarios had the same im-pact when developed by the person as when supplied byothers, it is clear that not everyone needs to be involvedin the scenario development process. However, one ben-efit of personal involvement is greater intellectual own-ership, so senior executives should be intimately in-volved in the process. Scenarios developed by othersmay have more surprise or learning value but perhapslower credibility.23But personal involvement may meanthat you bias the process or suppress new ideas. Biases in Scenarios Although scenarios can free our thinking, they can stillbe affected by biases. When we are making predictions,we tend to look for confirming evidence and discountdisconfirming evidence, and this bias can creep into thescenario development. I asked some MBA students todevelop both positive and negative scenarios for the in-dustries in which they expected to be employed aftergraduation, ranging from banking and managementconsulting to consumer products and real estate. (I con-ducted this study in 1986, when none of the studentswould have imagined that a stock market crash in 1987would adversely affect jobs on Wall Street and in man-agement consulting for several years after.) I also askedthem to score each trend as clearly positive, negative, orindeterminate. On average, each student identified twopositive trends in his or her field and only 1.48 negativetrends (see Table 5). They also weighted the probabilityof positive outcomes more heavily than negative out-comes.
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