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Weekly links January 18: an example of the problem of ex-post power calcs, new tools for measuring behavior change, plan your surveys better, and more...

David McKenzie's picture
  • The Science of Behavior Change Repository offers a repository of measures of stress, personality, self-regulation, time preferences, etc. – with instruments for both children and adults, and information on how long the questions take to administer and where they have been validated.
  • Andrew Gelman on post-hoc power calculations – “my problem is that their recommended calculations will give wrong answers because they are based on extremely noisy estimates of effect size... Suppose you have 200 patients: 100 treated and 100 control, and post-operative survival is 94 for the treated group and 90 for the controls. Then the raw estimated treatment effect is 0.04 with standard error sqrt(0.94*0.06/100 + 0.90*0.10/100) = 0.04. The estimate is just one s.e. away from zero, hence not statistically significant. And the crudely estimated post-hoc power, using the normal distribution, is approximately 16% (the probability of observing an estimate at least 2 standard errors away from zero, conditional on the true parameter value being 1 standard error away from zero). But that’s a noisy, noisy estimate! Consider that effect sizes consistent with these data could be anywhere from -0.04 to +0.12 (roughly), hence absolute effect sizes could be roughly between 0 and 3 standard errors away from zero, corresponding to power being somewhere between 5% (if the true population effect size happened to be zero) and 97.5% (if the true effect size were three standard errors from zero).”
  • The World Bank’s data blog uses meta-data from hosting its survey solutions tool to ask how well people plan their surveys (and read the comments for good context in interpreting the data). Some key findings:
    • Surveys usually take longer than you think they will: 47% of users underestimated the amount of time they needed for the field work – and after requesting more server time, many then re-request this extension
    • Spend more time piloting questionnaires before launching: 80% of users revise their surveys at least once when surveying has started, and “a surprisingly high proportion of novice users made 10 or more revisions of their questionnaires during the fieldwork”
    • Another factoid of interest “An average nationally representative survey in developing countries costs about US$2M”
  • On the EDI Global blog, Nkolo, Mallet, and Terenzi draw on the experiences of EDI and the recent literature to discuss how to deal with surveys on sensitive topics.

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