My data is some percentages on how many transactions from a whole are missed each month. The percentages are for 13 months and ranging from 97 to 99 percent. I was asked to calculate the mean and standard deviation, but I am unsure if the result would be meaningful and if the mean could be calculated for percentages in the traditional sense (e.g., in Excel doing AVERAGE(percent array)) vs. some other method (weighted averaged) since I do not have any other information other than the percentages.
Please help me to understand
- If the mean and standard deviation for percentages can be calculated,
- What conditions are needed in order to calculate the mean and standard deviation for percentages,
- Alternatives determining spread and central tendency for percentages over a period of time.
For example, would the following be incorrect to do assuming that the percentages represent # transactions missed / # total transactions for each different months with the same calculation being used for each month:
Month Data Feb-15 98.0% Mar-15 98.7% Apr-15 97.0% May-15 99.9% Jun-15 98.7% Jul-15 97.9% Mean 98.4% SD (Population) 0.90%
From this post is seems like it should be done with weighted averages since the totals are different, and that calculating the mean and standard deviation is possible for percentages only if they come from the same total, meaning the above would be incorrect and that I would need additional information to determine the weight to multiple the percentages by, if that is correct.