# QMS 202 Lecture Notes - Standard Deviation, Confidence Interval

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16 Apr 2012

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Summary of lecture 1

What is unknown? Population Mean and Population proportion.

What is known? Sample mean and sample proportion.

We can always take a sample of quantitative data (via survey)

and calculate the sample mean. We will use this sample mean to

construct an interval that will contain the population mean c% of

the time. This c% (i.e. 95%, 90%, 92% etc) indicates our

confidence that this interval will contain the (true) population

mean. We use this c% to calculate the z and t values (known as

critical values) where this c% is treated as "area under the

curve".

I have shown you two ways of calculating the confidence interval

for

:

1. Using the formula (refer to equation 10.1 on page 407 of the

textbook)

2. Using the calculator function "INTR" (on page 409)

The third way of calculating the confidence interval is to use

SPSS (software) which was not shown in class but the

instructions are in my powerpoint slides.

When you select "INTR" function, you will encounter "Z" and "t".

Select "Z" if population standard deviation is given in the

problem.

Select "t" if population standard deviation is NOT given in the

problem.

Z and t are the critical values.

To find Z values, use "INVN" function and input "area" (which is

the confidence level (usually expressed in %); std dev=1; and

mean=0.

To find the t values, you have two options: (1) t table (which will

be provided in the quiz/test/exam) and (2) INVt function and

input "upper area", "df" (degree of freedom=n-1).

Does this help?