When does a production function satisfy decreasing returns to scale?

Diminishing or decreasing returns to scale where the increase in output is more than proportionate increase in quantities of factor inputs it is called diminishing returns to scale.

Here, IQ₁, IQ₂ and IQ₃ represent the total output of 1000 units, 2000 units and 3000 units respectively.

As we can see EE₁ < E₁E₂ < E₂E₃, so it is decreasing returns to scale.