The answers appear in the lower frame when you click on answer . Links to the appropriate sections of the text are also included. Use it do show spread. Use it for normally distributed data. Indicate the middle of some data.
Dimension reduction describes loss of precision.
It is the correlation between true and observed values. A valid measure must be reliable. A reliable measure must be valid. A correlation coefficient and its confidence interval are shown in the figure. The true value of the correlation is likely to be 0.45.