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The median market practice line is a line of best fit, adjusted so that one half of the survey data points lie above it, and one half below it. This line is shown in Chart 4 as the central, more heavily drawn line.

Other commonly developed lines for market data are the third quartile and first quartile lines. The third quartile line has three quarters of the data points below it and one quarter above. The first quartile line has one quarter of the data points below it and three quarter above. These two additional lines, the third quartile and first quartile, are the more lightly drawn upper and lower lines respectively on Chart 4.

Chart 4

Just as there is a formula for the company practice line, there are also formulas for the market practice lines. For our market sample shown in Chart 4 the formulas are:

Median (Q2) Salary = Job Points x $150.38 + $10,918

First Quartile (Q1) Salary = Job Points x $137.01 + $9,947

Third Quartile (Q3) Salary = Job Points x $164.45 + $11,954

Note from Chart 4 that the first and third quartile lines diverge from the median as job size increases. This is because there is a greater spread in the absolute value of salaries as job size increases. This is true even though the proportionate spread may not increase, and is explained as follows: