IGCSEMaths Statistics & Probability.

Using frequency instead of frequency density for histograms

When class widths are unequal, the y-axis must show frequency density (frequency divided by class width). Using raw frequency gives bars with incorrect heights and loses all marks.

Not using the midpoint for estimated mean from grouped data

The estimated mean uses the midpoint of each class interval multiplied by its frequency. Using the upper or lower bound instead of the midpoint gives an incorrect answer.

Forgetting to multiply along branches of a tree diagram

To find the probability of a combined outcome, multiply the probabilities along the branch. Then add the probabilities of different branches that lead to the same overall outcome.

Not adjusting probabilities for "without replacement" questions

When items are not replaced, the total number decreases and the composition changes for the second selection. A common error is keeping the denominators the same as the first pick.

Misreading cumulative frequency as frequency

Cumulative frequency gives the running total up to each value. To find the frequency for a single class, subtract the previous cumulative frequency from the current one.