How do the values of SD and SEM, and the width of the 95% CI, relate to the range of the data? It is easier to show two values, or a graph with two dots, than to tabulate or graph the mean plus/minus error. You may not agree with that opinion with moderately large data sets, but with n=2, it really makes no sense. There are better alternatives to plotting either the SD or the SEM. You can display the actual data in the same amount of space. When you have fewer than say 100 values, there really is not much point in graphing a mean with SD or SEM. Why display mean and SD or SEM rather than the raw data? (But of course t tests and ANOVA cannot be done with n=1.) But n=2 is enough for the results to be valid. Is it valid to compute a t test or ANOVA with only two replicates in each group? The equations that calculate the SD, SEM and CI all work just fine when you have only duplicate (N=2) data. ![]() It seems to be common lab folklore that the calculations of SD or SEM are not valid for n=2. Is valid to calculate the SD or SEM or CI of two values? Which statistical calculations are valid when you only have two values in each group?
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