Help with Confidence Interval, please

[HistoryStudentGWU]

Hi, I'm so rusty at statistics (took it a year ago) I've got the basic formula down, but this particular problem is hitting me in a blind spot.

Any initial advice would be greatly appreciated!

Family-owned companies are notorious for having difficulties in transferring control from one generation to the next. Part of this problem can be traced to lack of a well-docuented strategic business plan. In a survey of 3,900 privately held faily firms with revenues exceeding $1,000,000 a year, Arthur Andersen found that 1,911 had no strategic business plan.

The QUESTION:

Assume the 3,900 firms were randomly sampled from the population. Use a 90% confidence interval to estimate the proportion of family-owned companies without strategic business plans.

[HistoryStudentGWU]

3. Confidence Interval for the Proportion, Large Samples

As long as n*p > 5 and n * q > 5, the sampling distribution of the proportion follows the normal distribution.

p +/- [ z * sp ]
where p = sample proportion
q = 1-p
sp = standard error of the proportion = sqrt(pq/n)
z = two-tailed z score for the desired level of confidence


Is this the correct formula for this problem?

[HistoryStudentGWU]

I came up with an interval of 47.7% to 50.3% proportion of companies who will not provide strategic biz plans.

Is this correct?

[JohnM]

Looks good.