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Two different types of injection–moulding machines are used to form plastic parts. A part is considered defective if it has excessive shrinkage or is discoloured. Two random samples, each of size 300, are selected and 15 defective parts are found in the sample from machine 1 whereas 8 defective parts are found in the sample from machine 2.

a) Test, at the 5% level of significance, if both machines produce the same fraction of defective parts. State your null and alternative hypotheses and the conclusion of the test in plain language.

b)Find a 95% confidence interval for the difference in the fraction of defective parts produced by the two machines. How does this confidence interval confirm the conclusion reached in part (a) above?

2. Relevant equations

3. The attempt at a solution

a) I found that there is no significant difference in the fraction of defective parts produced by the machine at the 5% level of significance.

I would put the working up here but it will take a while to type out...if you really want to see it I'll type it up.

Part b is where I am confused. Do I just do the same working as part a, but instead of using α=0.05 I use α=0.95?

Thank you.

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# Homework Help: Statistics - 95% Confidence Level

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