Perfect toy? You do the math.

Agonising over what to buy your child for Christmas?

Then hunt out your calculator - and try the formula below.

The equation created by psychologist Cliff Arnall is designed to calculate a toy's 'play value' based on factors including cost, the amount of time a child is expected to spend with it and whether it encourages social interaction.

A toy that scores 40 or above is considered to to be of excellent value.

Professor Arnall developed the equation after a study of 5,000 parents and children, commissioned by British toy company Worlds Apart, revealed that 65 per cent of youngsters receive Christmas presents they don't like or don't play with.

Parents are required to assess a toy against criteria such as creativity and whether it is durable enough to be handed down.

Each of the five criteria should be given a score out of five and then totalled.

Separately, the amount of time in hours the child is expected to play with the toy needs to be multiplied by the length of time in months the child is expected to keep it.

The result needs to be divided by the square root of the cost. This number is then added to the total marks the product scored in the criteria section.

Hmmm....While all for the math, I’ll take my chances with intuition. And what my eight year old tells me he wants.