Sample Size Still Matters
"Stand at the door of a church on a Sunday and bid 16 men to stop, tall ones and small ones, as they happen to pass out when the service is finished; then make them put their left feet one behind the other, and the length thus obtained shall be a right and lawful rod to measure and survey the land with, and the 16th part of it shall be the right and lawful foot." - Jacob Koebel 16th century
Does a sample size of 16 have the statistical power for credible assignment results?
If the sample distribution is normal, a minimum sample size of 15 is required. A normal distribution will have equal mean, median and mode.
If the sample distribution is non-normal, a minimum sample size of 30 is required.
“Regardless of the distribution of the population, the sampling distribution of the mean is approximately normal when sample size is 30 or more. In practice, the Central Limit Theorem allows us to make inferences about population means relying on the normal distribution when a) the population is normal or b) when n ≥ 30. As a practical matter, the sampling distribution of the mean will be approximately normal when n ≥ 15 and the population is symmetrically distributed. However, appraisers usually know very little about the shape of population distributions of price, property attributes, financing arrangements, and the like. Therefore, the n ≥ 30 criterion generally applies to real property valuation work.”
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