The Spread of Wage Rates

The lnterquartile Range
The median doesn't show the many different wage rates paid.

Some people have very high rates. Others have very low rates. One way to measure this spread is to look at the middle 50 per cent. We can use the interquartile range to compare men's wages and women's wages before and after the Equal Pay Act.

The middle 50 per cent runs from 25 per cent to 75 per cent. (See Figure 3.) 25 per cent of men earn less than £1.52 per hour.

Since 25 per cent is one quarter, this wage rate is called the first (lower) quartile and labelled Q₁ (Note that the quartile is read on the horizontal axis.)

Q₁ = £1.52 for men in 1978

75 per cent of men earn less than £2.37 per hour. Since 75 per cent is three-quarters, this wage rate is called the third (upper) quartile and labelled Q₃.

(The median is Q₂, the second quartile).

Q₃ = £2.37 for men in 1978

1. Find Q₁ (the first quartile) for women's wage rates in April 1978.
2. Find Q₃ (the third quartile) for women's wage rates in April 1978.

We can use Q₁ and Q₃ to ignore the 25 per cent of workers with the lowest pay and the 25 per cent of workers with the highest pay.

Q₃ - Q₁ is called the INTERQUARTILE RANGE.

Men's pay rates in April 1978 give:

Q₁ = £1.52 per hour
Q₃ = £2.37 per hour

The interquartile range is from £1.52 per hour to £2.37 per hour. This is a range of £0.85 per hour.

3. Calculate the interquartile range for women's wage rates in April 1978.

Quartiles in April 1975 are shown in Table 6. This was before the Equal Pay Act was passed.

These data are also shown in Figure 4.

Figure 4 - Wage rates (April 1975).

4. Draw the corresponding diagram for April 1978, using the quartiles calculated in c.
5. How does it differ from that of 1975
6. Does the Equal Pay Act appear to be working? Give one reason why we can't be sure.

Changes over Three Years
Here are some more figures to help you decide about the effect of the Act.

Table 7 - Quartiles of rates of pay, 1976/77

1. Find the interquartile range for men and for women in 1976.
2. Find the interquartile range for men and for women in 1977.
3. Write down what effect (if any) the Equal Pay Act has had between 1975 and 1978.