How to Spot Mathematical Errors in Statistical Data
After examining the design of the study and how data was collected, the next thing to do when you come upon a statistic or the result of a statistical study is to look for mathematical errors in the data. Start by asking, “Is this number correct?” Don’t assume it is! You’d probably be surprised at the number of simple arithmetic errors that occur when statistics are collected, summarized, reported, or interpreted.
To spot arithmetic errors or omissions in statistics:

Check to be sure everything adds up. In other words, do the percents in the pie chart add up to 100 (or close enough due to rounding)? Do the number of people in each category add up to the total number surveyed?

Doublecheck even the most basic calculations.

Always look for a total sample size so you can put the results into proper perspective. Ignore results based on tiny sample sizes.

Examine whether the projections are reasonable. For example, if three deaths due to a certain condition are said to happen per minute, that adds up to over 1.5 million such deaths in a year. Depending on what condition is being reported, this number may be unreasonable.