The curve is tallest there, so that exact score must be the biggest percent.
The student uses how tall the normal curve is at one value as the percent of people. They treat a single point as if it has area.
They see the curve like a bar graph. They miss that a percent comes from area under the curve across a range.
Words rarely shift this one. It needs something to look at — a picture, a fold, an object on the table — before the explanation lands.
Using the upper cutoff alone
What you might hear
It says 82% is below 74, so between 66 and 74 must be 82%.
For a question about values between two cutoffs, the student reports the percent below the upper cutoff. They do not subtract the part below the lower cutoff.
They know a table or calculator gives an area up to one value. They do not yet see that an interval needs two boundary areas.
This one is not a slip to correct — the underlying idea needs re-teaching before more practice will help.
Reporting the wrong tail
What you might hear
It asks above 90, but the table gives 91%, so the answer is 91%.
When the question asks for a value above a cutoff, the student gives the percent below that cutoff from the table or tool.
They treat the first area output as the answer. They do not check whether the question asks for the left side or the right side.
This one responds to correction in the moment — the idea is there, the rule being applied is not.
What to say when you see it
For each mistake above, Algo School gives you the words to say next — the specific response that corrects the thinking instead of just marking the answer wrong, plus practice aimed at that exact error.
A population is normally distributed with mean 80 and standard deviation 10. What is the z-score for a value of 60?
Answer-2.0
Generated by univariate-stats-alg2, a deterministic question engine — not written by an AI, and verified before any child sees it.
FAQ
Questions parents ask about this
How do I know which of these mistakes my child is making?
Look at what they SAY, not just what they wrote. Each mistake above includes the phrasing a parent typically hears — that sentence is usually the giveaway. In Algo School, the tutor recognises these patterns while your child is working and adapts the session around the one it sees.
Is "Use normal distributions to estimate population percentages" something children normally struggle with?
Yes. We track 3 distinct approved mistake patterns for this concept, each one written up because it recurs — not because it is unusual. A child making one of them is doing something systematic, which is far easier to fix than random errors.
What should I do once I know which mistake it is?
Respond to the thinking, not the answer. Algo School gives parents the specific wording for each mistake, then generates practice targeted at that error and shows you whether it actually cleared.
Where do these mistakes come from?
They are part of the same content library that drives practice: each one is reviewed and approved before it is used, and the Conditional Probability and Normal Distributions questions on this site are checked by an automated verification pass before a child is ever served them.
Related
Other places children go wrong
Where this sits
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