- degrees of freedom (
*df*) - the number of objects in a sample that are free to vary

- pooled proportion
- estimate of the common value of
*p*and_{1}*p*_{2}

- standard deviation
- a number that is equal to the square root of the variance and measures how far data values are from their mean; notation:
*s*for sample standard deviation and*σ*for population standard deviation

- variable (random variable)
- a characteristic of interest in a population being studied
Common notation for variables are uppercase Latin letters

*X*,*Y*,*Z*,... Common notation for a specific value from the domain (set of all possible values of a variable) are lowercase Latin letters*x*,*y*,*z*,.... For example, if*X*is the number of children in a family, then*x*represents a specific integer 0, 1, 2, 3, .... Variables in statistics differ from variables in intermediate algebra in two ways:- The domain of the random variable (RV) is not necessarily a numerical set; the domain may be expressed in words; for example, if
*X*= hair color, then the domain is {black, blond, gray, green, orange}. - We can tell what specific value
*x*of the random variable*X*takes only after performing the experiment.

- The domain of the random variable (RV) is not necessarily a numerical set; the domain may be expressed in words; for example, if