Calculation 95/5 Sampling

[jferrone]

I am trying and failing to replicate this calculation in Alteryx and Excel.

Your recommended sample size is: 225

The sample size (n) is calculated according to the formula: n = [z2 * p * (1 - p) / e2] / [1 + (z2 * p * (1 - p) / e2 * N)]

Where: z = 1.96 for a confidence level (α) of 95%, p = proportion (expressed as a decimal), N = population size, e = margin of error.

z = 1.96, p = 0.5, N = 542, e = 0.05

n = [1.962 * 0.5 * (1 - 0.5) / 0.052] / [1 + (1.962 * 0.5 * (1 - 0.5) / 0.052 * 542)]

n = 384.16 / 1.7088 = 224.815

n ≈ 225

The sample size (with finite population correction) is equal to 225

https://goodcalculators.com/sample-size-calculator/

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I have 4 Formulas in Alteryx at the moment and a constant for record count (542 in this case)

a2 = POW(1.96,2) 

e2 = POW(0.05,2)

Calculation P1 = ([a2]*0.5*(1-0.5)/[e2])

Calculation P2 = 1 + ([a2] * 0.5 * (1 - 0.5) / [e2] * [Count])

The first 3 calculations match the expected results but Calculation P2 is incorrect. I received similar results in Excel. The final result should be Calculation P1 / Calculation P2.

Any idea what I am doing wrong? 

[Claje · Accepted solution]

Hi,

I think you're just missing a set of parentheses around the denominator for Calculation P2 (I also think that the website this is coming from is missing the same parentheses).

If I run calculation P2 as 1 + [a2] * 0.5 * (1 - 0.5) /( [e2] * 542)

I get to the Final result of 225.

[Kenda · Accepted solution]

Hi @jferrone 

I think your denominator is just missing some parenthesis. For your Calculation P2, change it to 

1 + (([a2] * 0.5 * (1 - 0.5)) / ([e2] * [Count]))

 and see if that gets you what you're looking for. 

[jferrone]

Thank you both for your help, I appreciate the quick response 

[bbpogs]

Hi sir,

Hope you could share the updated workflow. 

[andrewsh]

in case someone else finds this like I did and got stumped by the answer.

((pow(1.96,2) * 0.5 * (1 - 0.5)) / (pow(.05,2))/
(1 + ((pow(1.96,2) * 0.5 * (1 - 0.5)) / (pow(.05,2) * 542))))

That is how it looks in alteryx formulas. 

there were a few more parentheses I had to add to make it work.