Sunday, January 27, 2013

p value bonferroni correction

For example, a researcher is testing 20 hypotheses simultaneously, with a critical P value of 0.05. In this case, the following would be true:
  • P (at least one significant result) = 1 – P (no significant results)
  • P (at least one significant result) = 1 – (1-0.05)20
  • P (at least one significant result) = 0.64
Thus, performing 20 tests on a data set yields a 64 percent chance of identifying at least one significant result, even if all of the tests are actually not significant. Therefore, while a given α may be appropriate for each individual comparison, it may not be appropriate for the set of all comparisons.

In general, if we have k independent significance tests at the α level, the probability p that we will get no significant differences in all these tests is simply the product of the individual probabilities: (1 - α)k. For example, with α = 0.05 and k = 10 we get p = 0.9510 = 0.60. This means, however, we now have a 40% chance that one of these 10 tests will turn out significant, despite each individual test only being at the 5% level. In order to guarantee that the overall significance test is still at the α level, we have to adapt the significance level α′ of the individual test.
This results in the following relation between the overall and the individual significance level:
(1 - α′)k = 1 - α.
This equation can easily be solved for α′:
α′ = 1 - (1-α)1/k,
which for small α reduces to:
α′ = α / k
This is a very simple recipe: If you want an overall significance level α and you perform k individual tests, simply divide α by k to obtain the significance level for the individual tests.

Thursday, January 24, 2013

linux basic system admin tutorial from IBM

install software pre-compiled or from source

Managing shared library

Basis System admin

All type of  work for System admin in linux

all type of linux problem solution

Wednesday, January 23, 2013

C/C++ basic tutorial

Memory allocation stack vs heap

Wednesday, January 2, 2013

Ghost script manipulate pdf files

1. Merger pdf files

gs -dNOPAUSE -sDEVICE=pdfwrite -sOUTPUTFILE=combinedpdf.pdf -dBATCH 1.pdf 2.pdf 3.pdf