'\"#ident	"@(#)shells:eqn.stats	1.3"
.H 1 "Measures of Central Tendency: Mean, Median and Mode"
.P
The mean is the arithmetic average for a set of scores.
The formula for computing a mean (M) is
.sp 2
.DS
.EQ
M ~=~ {sum from {i~=~1} to n {x sub i} } over n
.EN 
.DE
.P
The median divides ranked scores into halves.
Given that the \f2median interval\f1
is the score interval that contains the n/2nd largest score when scores 
are ordered by size, the formula for computing a median (Md) is
.sp 4
.DS
.EQ
Md ~=~ left ( pile {Lower~real above limit~of above median~interval}
       right ) 
       +
       left ( pile {width~of above median above interval}
       right )  
       left [ 
            {(n / 2) - left ( pile {Cumulative 
                                    above frequency~up\ to 
                                    above the~median~interval} 
                       right )} 
             over { pile {frequency~in above median~interval} }
        right ] 
.EN
.DE
.P
The mode is the most frequently occurring score in a group of scores.