# 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

Relevance

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.

• Anonymous
4 years ago

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