Confidence Intervals or P Values?

Statistical Principles: Confidence Intervals or P Values? Page 1/3

In a study comparing the effect of 2 cholesterol reducing drugs in patients with cardiovascular disease, drug A reduced the total cholesterol level in 40 patients by 0.2mmol/L with a 95% confidence interval for the reduction of 0.17 to 0.23. Drug B reduced the total cholesterol level in 25 patients by 0.3 mmol/L with a 95% confidence interval for the reduction of 0.22 to 0.38.

Welcome to the "Confidence Intervals or P Values?" quiz. There are 4 questions to answer.

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Q1. Which of the following options would be the most appropriate to use when reporting the results of a difference between the weight of two groups of subjects?

a)The mean difference was 5.5 kg (95%CI 5 – 6; p=0.02) ✔ ✗

b)The mean difference was 5.5 kg (95%CI ±0.5; p=0.02) ✔ ✗

c)The mean difference was 5.5 kg (95%CI 5.0 to 6.0; p=0.02) ✔ ✗

d)The mean difference was 5.5 kg (CI 5 to 6; p=0.02) ✔ ✗

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The answer is c). It is best to quote the confidence intervals to the same number of decimal places as the estimated parameter, the mean difference, and always use the word ‘to’ to distinguish between the lower and upper limits. Always quote the level of confidence, in this example 95%.

Q2. Which of the following options would be the most appropriate to use when reporting the results of a difference between the systolic blood pressure of two groups of patients?

a) The mean difference was 10 mmHg (95%CI -5 to 15) ; p=0.055) ✔ ✗

b)The mean difference was 10 mmHg (95%CI -5 to 15) ; p>0.05) ✔ ✗

c)The mean difference was 10 mmHg (95%CI -5 to 15) ; p=ns) ✔ ✗

d)The mean difference was 10 mmHg (95%CI -5 to 15) ; p=0.06) ✔ ✗

Feedback:

The correct answer is d). P values should be quoted to two decimal places and not replaced with ns or a greater than 0.05. In this example it was not significant at the 5% level, but only just, so this is useful information for the reader.

Q3. Which of the following statements are true?

a)If the 95% confidence interval for the difference in means is quoted as 0.5 to 1.0, the difference in means will be statistically significant. ✔ ✗

b)If the 99% confidence interval for a difference in proportions is quoted as -1.6 to -0.1, the difference in proportions will be statistically significant. ✔ ✗

c)If the confidence interval for an odds ratio is quoted as 0.4 to 1.3; The difference in exposure of the two groups of cases under consideration will be statistically significant. ✔ ✗

d)If the risk ratio is quoted as -2.3 to 0.9; the difference in exposure of the two groups of cases under consideration will be statistically significant. ✔ ✗

Feedback:

The correct answers are a) and b). a) If the confidence interval for a difference does not include zero then the difference will be statistically significant. b) If the confidence interval for a difference does not include zero then the difference will be statistically significant even it is a negative range. c) For risks and ratios, 1 is the value of no difference. A confidence interval that includes 1 indicates a lack of statistical significance for a risk or a ratio. d) This is a bit of a trick question. You cannot obtain a negative ratio. A confidence interval for a risk or a ratio will never cross zero, the lower limit value will be above zero.

Q4. Which of the following statements regarding the comparison of the two drugs A and B is correct?

a)The difference in the reduction in cholesterol level of the two drugs was not significant at the 5% level because the 95% confidence intervals for the two groups overlapped. ✔ ✗

b)The difference in the reduction in cholesterol level of the two drugs was significant at the 5% level because the 95% confidence intervals for the two groups did not overlap. ✔ ✗

c)None of the above statements ✔ ✗

Feedback:

The correct answer is c). a) Generally, when 95% confidence intervals for two sample means or proportions overlap, inferences cannot be made about the presence or absence of statistical significance at the 5% level. In this example the difference in the cholesterol reduction between drugs A and B was 0.10 mmol/L and the 95% confidence for the difference was (0.03 to 0.18) which does include zero, so the difference will be significant at the 5% level. b) The two confidence intervals overlap.

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