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What Are Null Events (Time-Slot holders)?
Generally speaking, a null event replaces the treatment event in a control group. Unlike a treatment, measurement, or support event, nothing happens when the program is executed and a Null Event is encountered. The purpose of inserting null events in protocols is to fill the time intervals in which other groups are presenting treatments (and sometimes measurements and support events). This aligns the events in one group with events in another thereby assuring that measurements and treatments are presented at the appropriate time in each group. This helps control for effects of history.
In some studies, instead of a null event, the event could be a placebo or other treatment. In this sense, a null event can be thought of as a "time-slot holder." If it is not important that treatment, measurement, or control events in one group are "aligned" (taken at the same time as their counterparts in other groups), then null events may not needed.
How to Create a Null Event
Since null events are generally associated with a treatment (or more accurately the absence of a treatment), we can create a null event as follows:
Click on the Events tab and above the Event Specification Table click on Create. Select "Treatment" and under that "Unspecified Treatment." Now, change the event label in the Event Specification Table from "Unspecified Treatment" to "Null Event." Fill out the event specification table for that event, scroll down to the Instrument Construction Area and simply describe this event as a substitute for a treatment event.
Immediately, or at a point in which you have created a number of events, you will enter the name of the null event in the control group into the Execution List Window. To do this, select the Control Group and select the null event in the Event Names Window. Then, click on the right arrow labeled "Enter Event Name."