----- Original Message -----
From: GEORGE PERKINS <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Thursday, April 27, 2000 10:12 AM
Subject: Degrees of Freedom
> I got a call the other day from a high school science teacher asking about
the following:
>
> She is testing different brands of yogurt for acid neutralization by
acidophilus bacteria. Her students have measured the pH of yogurt then
poured in a known amount of acid and began measuring pH in intervals of 1
minute for 5 minutes. She has six replicates for each of the types of yogurt
for a total of 12 time series. She wants to test if the mean concentration
of acid is different in the two groups by taking the initial pH value -
final pH value for each replicate getting a total of six differences per
group then finds a mean of differences for each set. Finally, she wants to
take the means from each set of differences and do a hypothesis test mu1=mu2
using a t-test but can't figure out the degrees of freedom of the test and
frankly I am not quite sure either. Her idea is to take 12-2 degrees but
others have said it should be 6-1 degrees. I wonder if others out there can
shed light on three issues:
............................................................................
............................
There is no relation between pH and acid concentration in terms of
moles/liter. pH is dependent on the H+ ionization constants of the
individual components. pH is just a measure of the H+ concentration, not of
the acid concentration.
DAHeiser
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