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August 5th, 2016, 08:50 PM   #1
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sample size determinant

Hi everyone,

I'm trying to determine a sample size. I am listening to calls to a Contact Centre from customers with Internet Banking or a banking app. I'll be listening for 3 different areas.

area 1 gets 3.3 million calls annually
area 2 gets 1.3 million calls annually
area 3 gets 800k calls annually.

40% of customers who call, have internet banking or banking app. Once I've listened to the calls I'll categorize what the call was about. Is there a model or algorithm that can help me determine how many calls I should listen to in each area to get, say a 95% confidence interval, for using my sample as a representation of the whole population?

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August 6th, 2016, 03:31 PM   #2
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You need an accuracy requirement for your result. 95% confidence is about 2 standard deviations from the mean. You need to specify how far from the mean is acceptable.
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August 7th, 2016, 09:05 PM   #3
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No mean

What would the mean be tho? If I was taking the mean age of each person who responded to a survey then I could provide one but I'm looking to get a good sample representation for call types for which they'd be no mean.

What do people do in cases of non-parametric data?

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August 8th, 2016, 01:07 PM   #4
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I am somewhat confused. What defines call types? What are you trying to represent?
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August 8th, 2016, 09:59 PM   #5
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Hey Mathman,

Call types are defined by people who call. For example say you get 10000 calls a year 3000 might be about Credit Cards, 2000 about loans, 4000 about accounts and 1000 about general inquiries. It depends on why the customer is calling, only way to know is to listen to the call and record the information.

We get about 4.4 million calls a year and I want to listen to a sample of those so I can make judgments on future changes. e.g. Credit card calls are now going to be 50% cheaper and they cost $4 per call, how much would we save. So $2*3000 = $6000.

That's a simplistic and unrealistic overview but essentially what I'm trying to do; get a sample size that represents the population.

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Scott
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August 9th, 2016, 01:59 PM   #6
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If I understand you correctly, the mean you are trying to estimate is the number of calls of a given type?

Using your credit card numbers, $\displaystyle \sigma = \sqrt{npq}=\sqrt{10000\times .3\times .7}=45.8$. Your 95% confidence interval is $\displaystyle 3000\pm 91.6$.
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