## 8.1 Quick revision

We recommend trying these Quick revision questions before your tutorial.

Researchers studied the impact of a gluten-free diet on dental cavities . Some of the summary information regarding the number decayed, missing and filled teeth (DMFT) is shown below.

Diet Sample size Mean Standard deviation Standard error
Coeliac 23 8.39 4.4 0.92
Non-coeliac 23 8.17 4.1 0.86
Difference 0.22   1.3

An exact 95% CI is given as for the difference is $$-2.32$$ to $$2.76$$.

1. Using the 68--95--99.7 rule gives a slightly different CI. Why?
2. The difference is computed as the number of DMFT for coeliacs minus non-coeliacs. True or false?
3. One of the values for the CI is a negative value, which must be an error, since a negative number of DMFT is impossible. True or false?
4. We are 95% confidence that the difference between the population means is:
1. The researchers used an exact 95%CI, which gives very similar answers to using an approximate 95% CI. (The 68--95--99.7 rule gives approximate multipliers only.)
2. TRUE. If we take the coeliac mean minus the non-coeliac mean, we would have $$8.39 - 8.17 = 0.22$$, which is what is given in the table.
3. FALSE. The CI is for the difference in the DMFT. The value is not about the number of teeth; it is about the difference in the number of teeth between coeliacs and non-coeliacs. A negative value is fine if we understand what the CI is estimating.
4. The positive value of 2.76 means (as we saw in the first Quick Revision question) that coeliacs have a mean of 2.76 more DMFT.
So the negative value of $$-2.32$$ means that non-coeliacs have a mean of 2.32 more DMFT.

### References

Khalaf ME, Akbar A, Alkhubaizi Q, Qudeimat M. Caries among adult patients with controlled celiac disease: A cross-sectional study. Special Care in Dentistry. Wiley Online Library; 2020;40(5):457–63.