To find out the concentration of SO₂ (in parts per million, i.e., ppm), in the air, the data was collected for 30 localities in a certain city and is presented below :

Concentration of SO2 (in ppm)	Frequency
0.00 - 0.04	4
0.04 - 0.08	9
0.08 - 0.12	9
0.12 - 0.16	2
0.16 - 0.20	4
0.20 - 0.24	2

Find the mean concentration of SO₂ in the air.

We will use step deviation method to find the mean.

In the following table a is the assumed mean and h is the class size.

Here, h = 0.04

By formula,

Class mark = $\dfrac{\text{Upper limit + Lower limit}}{2}$

By formula,

Mean = a + $\dfrac{Σf$iui}{Σf_i} \times h

Substituting values we get :

$\text{Mean } = 0.10 + \dfrac{-1}{30} \times 0.04 \\[1em] = 0.10 - \dfrac{0.04}{30} \\[1em] = 0.10 - \dfrac{4}{3000} \\[1em] = 0.10 - \dfrac{1}{750} \\[1em] = 0.10 - 0.00133 \\[1em] = 0.099$

Hence, mean concentration of SO₂ = 0.099 ppm.