The chi-squared (c2) test is a measure of expected frequency versus observed frequency to determine how closely aligned they are; whether what is observed would be expected or whether it is different to what would be expected.
The default expectation is the null hypothesis which states that no difference exists between the things we are experimenting with, observing or comparing and their default, or control state.
The chi-squared value we obtain from the chi-squared test is put against a probability scale. The probability refers to the probability that for a given chi-squared value (which is defined by how different our observation is from what we would expect), what we observe really is different to what we expect to a significant extent that enables us to reject the null hypothesis.
For example, our expected values for the eye colour of 154 flies in a population would be…