Jeff_Kaufman comments on Independent re-analysis of MFA veg ads RCT data - Effective Altruism Forum

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Comment author: Jeff_Kaufman 20 February 2016 06:08:47AM *  3 points [-]

it looks like you're using a one-sided t-test to get your p-value.

I agree that a two-sided test would be the right thing to use here, and p-value calculations aren't something I fully understand. Is this calculation one-sided or two-sided?

Comment author: MichaelDickens  (EA Profile) 20 February 2016 04:33:18PM *  4 points [-]

It looks like the NORMDIST function on your sheet is taking the integral from 0 to z_score, which is one-sided. A two-sided test would take

(integral from 0 to z_score) + (integral from -z_score to infinity)
Comment author: Dan_Keys 20 February 2016 06:50:27AM 2 points [-]

I can't tell what's being done in that calculation.

I'm getting a p-value of 0.108 from a Pearson chi-square test (with cell values 55, 809; 78, 856). A chi-square test and a two-tailed t-test should give very similar results with these data, so I agree with Michael that it looks like your p=0.053 comes from a one-tailed test.

Comment author: Jeff_Kaufman 20 February 2016 02:46:41PM *  1 point [-]

Yes, you're right. Sorry! I redid it computationally and also got 0.108. Post updated.