Most survival analysis models for time-to-event data, like Cox regression, assume independence. The survival time for one individual cannot influence the survival time for another. This assumption doesn’t hold in many study designs. You may have animals clustered into litters, matched pairs, or patients in a multi-center trial with correlated survival times within a center.

When you hear about multilevel models or mixed models, you very often think of a nested design. Level 1 units nested in Level 2 units, which are in turn possibly nested in Level 3 units. But these variables that define the units and that become random factors in the model can, in fact, be crossed […]

There’s no mincing words here. Missing values can cause problems for every statistician. That’s true for a lot of reasons, but it can start with simple issues of choices made when coding missing values in a data set. Here are a few examples. Example 1: The Null License Plate Researcher Joseph Tartaro thought it would […]

When you learned analysis of variance (ANOVA), it’s likely that the emphasis was on the ANOVA table, with its Sums of Squares and F tests, followed by a post-hoc test. But ANOVA is quite flexible in how it can compare means. A large part of that flexibility comes from its ability to perform many types […]

In part 2 of this series, we got started on the various menus in Stata. This post covers an important menu that you’ll probably use often: the graphics menu. What’s in the Graphics menu The graphics menu provides an impressive variety of options for creating just about any graph you might need. Take a look […]

How do you know which method to use when you want to compare groups?

The classic way to compare means in analysis of variance is examining pairwise differences in means after an F test.. It’s great for many problems, but sometimes your research question isn’t about pairwise differences. Pairwise differences are not ideal if your research question is if, like the Sesame Street song, one of these groups is […]

A great way to get started with Stata is using its menus. The first part of this Tutorial Series introduced you to Stata’s windows.  You can now begin learning how to use Stata to work with data. Across the top are 8 tabs: File, Edit, Data, Graphics, Statistics, User, Window, and Help. We will not […]

One of the hardest steps in any project is learning to ask the right research question!

Designing experiments would always be simple if we could just randomly assign subjects to different treatment conditions with no other restrictions. Unfortunately, that doesn’t always work. For example, there are many experimental situations where the subjects aren’t independent of each other. The subjects that are related to each other are combined into clusters called “blocks.” […]