• #### Q what re-sampling could help t-test

From Cosine@21:1/5 to All on Tue Jan 7 11:32:24 2020
Hi:

When doing analysis for problems with a small sample, a popular way is to replace the z-test to t-test. However, there is still another approach, the re-sampling method. One can repeatedly and randomly draw "new" samples from the original sample set to
form another sample set. After a set fo "new" sample sets are built, one can do analysis on these sample sets. But what does this type of approach helps to "cure" the problem of having a small sample set? Does it help improve the power of analysis or
else?

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• From Rich Ulrich@21:1/5 to All on Tue Jan 7 16:38:42 2020
On Tue, 7 Jan 2020 11:32:24 -0800 (PST), Cosine <asecant@gmail.com>
wrote:

Hi:

When doing analysis for problems with a small
sample, a popular way is to replace the z-test to t-test.
However, there is still another approach, the re-sampling
method. One can repeatedly and randomly draw "new"
samples from the original sample set to form another
sample set. After a set fo "new" sample sets are built,
one can do analysis on these sample sets. But what
does this type of approach helps to "cure" the
problem of having a small sample set? Does it help
improve the power of analysis or else?

What you are describing is called "bootstrap".
It is used for circumstances where the direct computation
of the variance is hard to define, or is made unreliable by
oddities of the distribution.

The t-test is simple. Thus, it is not improved by bootstrapping.

The choice between assuming "common variance" and
"separate variances" for the two groups should depend
on expectations based on expectations a professional in
the area would have for the data, /not/ on the test (in SPSS,
say) that tells you that "variances are unequal."

The biggest help for robustness of t-testing is the
willingness to perform a transformation that produces
a scale that is "interval" in terms of whatever the
hypotheses are about. (For instance: chemical
concentrations in biological processes are typically
compared as "twice as much" or "ten times as much" -
implying that /those/ processes merit taking the logs
of the raw concentrations, to produce "equal intervals."

--
Rich Ulrich

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• From David Jones@21:1/5 to Rich Ulrich on Wed Jan 8 00:26:34 2020
Rich Ulrich wrote:

On Tue, 7 Jan 2020 11:32:24 -0800 (PST), Cosine <asecant@gmail.com>
wrote:

Hi:

When doing analysis for problems with a small
sample, a popular way is to replace the z-test to t-test.
However, there is still another approach, the re-sampling
method. One can repeatedly and randomly draw "new"
samples from the original sample set to form another
sample set. After a set fo "new" sample sets are built,
one can do analysis on these sample sets. But what
does this type of approach helps to "cure" the
problem of having a small sample set? Does it help
improve the power of analysis or else?

What you are describing is called "bootstrap".
It is used for circumstances where the direct computation
of the variance is hard to define, or is made unreliable by
oddities of the distribution.

The t-test is simple. Thus, it is not improved by bootstrapping.

The choice between assuming "common variance" and
"separate variances" for the two groups should depend
on expectations based on expectations a professional in
the area would have for the data, not on the test (in SPSS,
say) that tells you that "variances are unequal."

The biggest help for robustness of t-testing is the
willingness to perform a transformation that produces
a scale that is "interval" in terms of whatever the
hypotheses are about. (For instance: chemical
concentrations in biological processes are typically
compared as "twice as much" or "ten times as much" -
implying that those processes merit taking the logs
of the raw concentrations, to produce "equal intervals."

The OP's question, being vague, encompasses also the possibility of
doing permutations for testing rather than bootstrapping (for variances
or testing). The same sort of problems outlined for bootstrapping still
apply, but there are rather fewer variations-on-a-theme for
permutations when it comes to constructing formal tests. In addition,
with permutations, it is rather clearer what role is being played by
the (what should be) explicit assumption that "all permutations are
equally likely" under the null hypothesis. Thus one would not consider
using permutations for an non-paired-two-group case, if there were not
strong evidence that variances in the two samples were equal. Similarly
the role of "pairs" in a permutation test for paired-sample two-group
tests is made strongly evident, in saying that the values in a pair may
be swapped or not-swapped with equal probability under the null
hypothesis.

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