#35
Top 50 Mistake
Confusing expected frequency with probability
"The Expected Error"
The Mistake in Action
A coin is flipped 50 times. The probability of heads is 0.5. How many heads are expected?
Student writes: "Expected heads = 0.5"
Why It Happens
Students give the probability as the answer when asked for expected frequency. They don't multiply by the number of trials.
The Fix
Expected frequency = Probability × Number of trials
Expected heads = $0.5 × 50 = 25$
The probability (0.5) tells you the proportion you expect. Multiply by the number of trials to get the actual count.
Think of it as: "If I flip 50 times and expect half to be heads, that's 50 × 0.5 = 25 heads"
Spot the Mistake
Can you identify where this student went wrong?
P(heads) = 0.5
Number of flips = 50
Expected heads = 0.5
Click on the line that contains the error.
Related Topics
Learn more about the underlying maths: