Statistics calculators

Statistics is one of those topics where the tools you need depend on whether you're analysing a dataset, doing a probability problem, or just sanity-checking a number you read somewhere. The tools here cover the descriptive measures (mean, median, mode, standard deviation), one inferential standardiser (z-score), and the two counting formulas that come up in every probability problem (permutations and combinations).

The mean / median / mode tool accepts any list of numbers (paste from a spreadsheet column, comma-separated, or one per line) and returns all three central tendencies plus quartiles and range. The standard deviation calculator shows population and sample formulas side by side because the divisor difference (n vs n-1) is one of the most-confused points in introductory stats. The z-score calculator standardises a value against a known mean and standard deviation, useful for comparing test scores or detecting outliers.

For counting problems, the permutations and combinations calculator handles both ordered (P(n,r)) and unordered (C(n,r)) selection, with a worked example showing why "5 choose 3 ice cream flavours" gives 10 and "first/second/third place from 5 runners" gives 60. These tools are not a replacement for R, Python, or a proper statistics package — they're for the moments when you need a number now and writing a script would take longer than reading the answer here.