A study was conducted at a major north-eastern American medical centre regarding blood cholesterol levels and heart-attack incidents. A total of 28 heart-attack patients had their cholesterol levels measured two days and four days after the attack. The units of measurement are milligrams per decilitre (mg/dL). The data are given below.

Investigate whether there is evidence that the mean cholesterol level two days after a heart attack is different from that four days after a heart attack.

1. State the model behind the appropriate t-test, and assess those assumptions for which suf?cient information has been provided. (You are given that your assessment will not reveal any problems with the model and that the t-test is appropriate.)

2. Clearly state the appropriate null hypothesis and carry out the t-test

3. If it is concluded that there is evidence of a difference between the mean cholesterol level two days after a heart attack and that four days after a heart attack, obtain a 95% con?dence interval for this difference. Explain in your own words how the con?dence interval should be interpreted.