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# how to you find degrees of freedom (z-table)?

this is my problem for stats:

a disk manufacturer wishes to know the fraction of disks his companies makes which are defective.

suppose a sample of 1527 floppy disks is drawn and of those 122 were defective. Construct an 85% confidence interval for the population proportion of disks which are defective

oh and the proportion = ,080

### 3 Answers

- 1 decade agoFavorite Answer
finding the degrees of freedom is a very easy thing. it is usually N-1. That is the total sample population minus 1. However in this case the degrees of freedom are not necessary. U are making a confidence interval right? This problem is whats known as a 1 proportion z test. If u have a TI 83 graphing calculator this problem is very easy. You go to STAT then TESTS and then scroll down until you find 1 prop z int. You plug in the info and it will calculate for u. If you do not have one the problem is a bit more work. first u will need to find the z test staistic for the 85 percent confidence level. DO this by looking up 85 percent on table. Looking it up on my table i see that it is between .8485 and .8508. This correpsonds to the z test statistic 1.13 and 1.14. Find the avg between these two numbers which is 1.135. This is ur z test statistic. The confidence interval is found by:

the statistic( 122/1527) plus/minus 1.135 times the standard deviation. the standard deviation for a proportion is found by the square root of (the statistic X (1-statistic) divided by the total sample size(1527)).

Hope this helps.

- Anonymous5 years ago
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RE:

how to you find degrees of freedom (z-table)?

this is my problem for stats:

a disk manufacturer wishes to know the fraction of disks his companies makes which are defective.

suppose a sample of 1527 floppy disks is drawn and of those 122 were defective. Construct an 85% confidence interval for the population proportion of disks which...

Source(s): find degrees freedom table: https://shortly.im/UvUwl - 5 years ago
P value is a statistical measure that helps scientists determine whether or not their hypotheses are correct. P values are used to determine whether the results of their experiment are within the normal range of values for the events being observed. Usually, if the P value of a data set is below a certain pre-determined amount (like, for instance, 0.05), scientists will reject the "null hypothesis" of their experiment - in other words, they'll rule out the hypothesis that the variables of their experiment had no meaningful effect on the results. Today, p values are usually found on a reference table by first calculating a chi square value

Source(s): https://tr.im/mI864