The June 2015 GCSE Subject Level Conditions and Requirements for Mathematics includes (P3)
“relate relative expected frequencies to theoretical probability, using appropriate language and the 0 – 1 probability scale”
and this leads to questions like
“If you rolled a die 600 times, how many sixes would you expect to get’.
which is taken from the CIMT MEP Pupil’s textbook on probability, and is given the answer
‘You would expect to get a 6 in 1/6 of the cases, so 100 sixes’.
This seems a confusing and misleading term. What exactly is an ‘expected frequency?’ The obvious meaning is the frequency that you expect. But we are trying to support the concept of a random variable, with ideas that a random variable is unpredictable in terms of value, that values do not form patterns or sequences, and can only be forecast and predicted in some general ways.
If you roll a die 600 times, I do not expect any value for the number of sixes. That is the most significant aspect of a random variable.
The implied sub-text is that
Expected frequency = probability X number of trials
So that, for example, if we toss a fair coin 100 times, what is the expected frequency of heads? Well, 50. So does that mean we expect to get 50 heads? This is a Bernoulli trial, and the probability of getting precisely 50 heads in 100 tosses is about 0.08. So we would need to say to a pupil
‘The expected frequency is 50; but it is unlikely that you would get 50 heads’
which hardly makes sense.
The probability of 51 is about .078, and 52 is .074. So, of course, 50 is the most likely frequency.
The phrase ‘most likely frequency’ is straight-forward, makes sense, and says what it means, unlike ‘expected frequency’.
Please can we stop using the phrase ‘expected frequency’?
De Morgan Gazette 