Ashwini Joshi

Ashwini Joshi is a subject matter expert in mathematics at Cytel. She received an MSc from IIT Bombay and has been an Assistant Professor in Mathematics at the PVG College of Engineering and Technology.

Recent Posts

How to handle conservativeness of Exact p-value?

Posted by Ashwini Joshi

Sep 18, 2017 7:38:45 AM

 

By Ashwini Joshi

For small sample data or rare events data, exact non-parametric tests perform better than asymptotic tests. But they come with the disadvantage of conservativeness. Many corrections have been suggested to reduce this conservativeness but none of them solve the problems entirely.  StatXact provides various methods of computing exact p-values. Depending upon the problem at hand, the user can decide which one to use.
Let’s consider a hypothetical example of stratified count data. The example shows two sample data with two strata. Events in Treatment1 are rare as compared to the ones in Treatment0. But the event rates are comparable.

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Topics: StatXact, Exact Tests, exact statistics, Small Sample Sparse Data, biostatistics, p value


Why and When to Use Profile Likelihood Based Confidence Intervals

Posted by Ashwini Joshi

Jul 14, 2015 11:00:00 AM

When conducting Maximum Likelihood Estimation, it is assumed that the maximum likelihood estimate follows a normal distribution. However, this may not be true in the case of small sample or sparse data.

Since the standard errors of the general linear model are based on asymptotic variance, they may not be a good estimator of standard error for small samples. In particular, Wald Confidence Intervals may not perform very well. One should only use the Wald Confidence Interval if the likelihood function is symmetric about the MLE.

 
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