4.5 Drills: Computational exercises

(Answers are available in Sect. A.5)

These Drill exercises (repeated practice) give you practice at getting computations correct, and using your calculator. These drill questions are more about practising the underlying mathematics rather than the statistics.

If you need help, please ask.

A study of drivers in (McEvoy et al. 2006) in New South Wales (NSW) and Western Australia (WA) were asked if they use their phone while driving (Table 4.3).
1. What percentage of people use their phone while driving?
2. What are the odds that a person uses their phone while driving?
3. What are the odds that a WA resident uses their phone while driving?
4. What are the odds that a NSW resident uses their phone while driving?
5. What are the odds ratio that a person uses their phone while driving, comparing WA residents to NSW residents?

TABLE 4.3: The number of drivers who use their phone while driving
	Uses phone	Does not use phone
NSW	381	295
WA	345	326

The data in Table 4.4 give the number of plum cuttings that were alive or dead, and whether the rootstock was long or short (Bartlett 1935).
1. Compute the percentage of cuttings that were alive at the end of the study.
2. Compute the percentage of cuttings that were dead at the end of the study.
3. Compute the percentage of cuttings that were long.
4. Compute the odds that a cuttings was alive at the end of the study.
5. Compute the odds that a cuttings was alive at the end of the study, only for the long cuttings.
6. Compute the odds that a cuttings was alive at the end of the study, only for the short cuttings.
7. Compute the odds ratio that a cutting was alive at the end of the study, comparing the short cuttings to the long cuttings.

TABLE 4.4: The relationship between length and condition of plum rootstocks
	Alive	Dead
Long	240	240
Short	138	342

References

Bartlett MS. Contingency table interactions. Supplement to the Journal of the Royal Statistical Society. JSTOR; 1935;2(2):248–52.

McEvoy SP, Stevenson MR, Woodward M. Phone use and crashes while driving: A representative survey of drivers in two Australian states. Medical Journal of Australia. 2006;185(11–12):630–4.