>>14213298If you check out
>>5942502, you'll find
https://sites.google.com/site/scienceandmathguide/, which leads to
https://sites.google.com/site/scienceandmathguide/subjects/mathematics#TOC-E-Books. I would advise you to check one of the statistics books listed there. Alternatively, I believe Khan Academy has decent introductory videos, though they will never give you the depth of knowledge of a well written textbook.
A standard deviation is a measure of spread of a distribution. In the case of normally distributed data, this gives rise to the well-known empirical rule, which states that 68% of the data will be within 1 standard deviation of the mean, 95% will be within 2, and 99.7% will be within 3.
The formula on the left of the image is for the sample standard deviation, which is used when attempting to estimate the standard deviation of the whole population from a limited sample thereof. If the numbers in the box were the entire population, you would use the formula for population standard deviation, which has $n$ on the bottom instead of $n-1$.
