17.12 Water diet. Water consumption before a meal has been shown to help reduce

Question

17.12Water diet. Water consumption before a meal has been shown to help reduce the caloric intake from that meal, but does it translate into actual weight loss? An experiment randomly assigned overweight and obese individuals aged 55 to 75 years to one of two diet groups for 12 weeks: a hypocaloric diet supplemented with 16 ounces of bottled water prior to each of the three daily meals (water group) or a hypocaloric diet alone (nonwater group).9

(a)Each subject had his or her body mass index (BMI) computed before and after the 12-week diet. For the 23 subjects assigned to the water group, the mean BMIs before and after treatment were 32.16 and 29.5, respectively. Explain briefly why a matched pairs procedure must be used to assess whether the combination of a hypocaloric diet and pre-meal water intake significantly lowers BMI.

(b)The 25 patients assigned to the nonwater group had mean BMIs before and after the 12-week diet of 31.8 and 29.9, respectively. Explain briefly why we cannot use a matched pairs procedure to test the hypothesis that the combination of a hypocaloric diet and pre-meal water intake is significantly better than a hypocaloric diet alone at lowering BMI.

Explanation / Answer

a)

As the data for mean BMI before and after the treatment is obtained from the same set of 23 subjects, it means that the two set of data are no more independent. So, we can't use independent sample t-test.

Rather these data are taken from the same set of people which makes it a paired data.

So, we use a paired data t-test.

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b)

Although the same 25 patients are used to obtain the data, but there is typically no treatment given to them. So, each of them must behave normally.

In this case, we can't use a paired data t test as there is no treatment. We can't take the different of their individual scores to use paired t test.

So, we just use the whole mean.

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