Power Calculations in Stata

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Guidelines

What data do I need?

You must have:

• Mean and variance for outcome variable for your population
  • Typically can assume mean and SD are the same for treatment and control groups if randomized

• Sample size (assuming you are calculating MDES (δ))
  • If individual randomization, number of people/units (n)
  • If clustered, number of clusters (k), number of units per cluster (m), intracluster correlation (ICC, ρ) and ideally, variation of cluster size

• The following standard conventions
  • Significance level (α) = 0.05
  • Power = 0.80 (i.e. probability of type II error (β) = 0.20

Ideally, you will also have:

• Baseline correlation of outcome with covariates
  • Covariates (individual and/or cluster level) reduce the residual variance of the outcome variable, leading to lower required sample sizes
    • Reducing individual level residual variance is akin to increasing # obs per cluster (bigger effect if ICC low)
    • Reducing cluster level residual variance is akin to increasing # of clusters (bigger effect if ICC and m high)
  • If you have baseline data, this is easy to obtain
    • Including baseline autocorrelation will improve power (keep only time invariant portion of variance)

• Number of follow-up surveys

• Autocorrelation of outcome between FUP rounds

How do I get this data?

You will basically never have the data you need for your exact population of interest at the time when you first do power calculations.

You will need to use the best available data to estimate values for each parameter. Sources to consider:

• High-quality nationally representative survey (e.g. LSMS)
• Data from DIME IE in same country (or region, if pressed)
• Review the literature – especially published papers on the sector and country. What kind of effects? Summary stats available?

If you can’t come up with a specific value you feel very confident in, run a few different power calculations with alternate assumptions and create bounded estimates.

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This article is part of the topic Sampling & Power Calculations

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