1. A Little Practice of everything
What should I do now?
A Little Practice of everything

2. Example:
There is some indication in medical literature that doctors become more aggressive in inducing labor or doing preterm caesarean sections when a woman is carrying twins. Records at a large hospital show that of the 43 sets of twins born in 1990, 20 were delivered before the 37th week of pregnancy. In 2000, 26 of 48 sets of twins were born preterm. Does this indicate and increase in the incidence of early births in twins? Test an hypothesis and state our conclusion.
2 proportion z - test

3. Researchers were interested in comparing two methods for estimating tire wear. The first method used the amount of weight lost by a tire. The second method used the amount of wear in the grooves of the tire. A random sample of 16 tires was obtained. Both methods were used to estimate the total distance traveled by each tire. The table below provides the two estimates (in thousands of miles) for each tire.
One sample t- test (matched pair)
Is there statistical evidence to suggest that the estimates of the two methods are different?

4. A study of chromosome abnormalities and criminality examined data from 4,124 males born in Copenhagen. Each man was classified as having a criminal record or not, using the registers maintained in the local police offices. Each was also classified as having the normal male XY chromosome pair or one of the abnormalities XYY or XXY. Of the 4,096 men with normal chromosomes 381 had criminal records, while 8 of the 28 men with abnormal chromosomes had criminal records. Some experts believe chromosome abnormalities are associated with increased criminality.
Construct and interpret a 95% confidence interval for the differences. Do these data lend support to this belief?
2 proportion z-interval

5. When patients undergo surgery, the operating room is kept cool so that the physicians in heavy gowns will not be overheated. The patient may pay the price for the surgeon’s comfort. The exposure to cold, in addition to impairment of temperature regulation caused by anesthesia and altered distribution of body heat, may result in mild hypothermia (approx 2o C below the normal core body temp). As a result of the hypothermia, patients may have an increased susceptibility to wound infection or eve heart attacks. In 1996, researchers in Austria investigated whether maintaining a patient’s body temperature close to normal by heating the patient during surgery decreased wound infection rates. Patients were assigned at random to two groups: the normothermic group (patients’ core temp were maintained at near normal 36.5o C with heating blankets) and the hypothermic group (patients’ core temp were allowed too decrease to about 34.5o C). If keeping patients warm during surgery reduces the chance of infection, then patients in the normothermic group should have shorter hospital stays than those in the hypotheric group. Here are summary statistics on length of hospital stay for the two treatment groups.
Two-sample t test

6. Two sample t interval

7. Population n X(natural) 1 (rural) 160 64 2 (urban) 261 89
Respondents who had a tree during the holiday season were asked whether the tree was natural or artificial. Respondents were also asked if they lived in an urban area or in a rural area. Of the 421 households displaying a Christmas tree, 160 lived in rural areas and 261 were urban residents. The tree growers want to know if there is a difference in preference for natural trees versus artificial trees between urban and rural households. Here are the data:
Population n X(natural)
1 (rural)
2 (urban)
Use an appropriate statistical procedure to answer the tree growers’ question.
Two proportion z-test

8. An association of Christmas tree growers in Indiana sponsored a sample survey of 500 Indiana households to help improve the marketing of Christmas trees. One question the researchers asked was, “Did you have a Christmas tree this year?” Of the 500 respondents, 421 answered “Yes.”
Construct and interpret a 90% confidence interval for the proportion of Indiana households who had a Christmas tree last year.