Demystifying PPM Sample Terminology
September 5, 2013
PPM isn’t exactly new on the scene, but are there some terms that you just never quite wrapped your brain around? Let’s see if we can clear things up. Open up your eBook and let’s dig in.
We’ll be working in the Market Info tab of the eBook, in the Population Estimates & Sample Summary section. Along the left-hand side, click on Sample Summary.
Out of all the people installed in the panel, what really matters is the people who are in-tab – these are the people who follow all the rules in order to be included in the estimates. So how many installed panelists are in-tab? That question can be answered by looking at the in-tab rate. You’ll find two in-tab rates – total persons and average daily persons. The total persons in-tab rate tells you how many people installed in the panel over the entire month were included in the estimates for at least one day. This will include people who left or came into the panel during that month. The average daily persons in-tab rate tells you, on an average day, how many people installed in the panel were included in the estimates for that day. As an example, if 2,000 people were installed on an average day and 1,700 people were in-tab on an average day, the average daily in-tab rate would be 85.0%.
Designated Delivery Index (DDI)
Okay, so the in-tab rate is 85.0%. Is that good or bad? This is where the Designated Delivery Index, or DDI, comes in. In each PPM market, Arbitron sets a target for how many people they would like to be in-tab on an average day. DDI compares the actual average daily in-tab to the target average daily in-tab. A DDI of 100 or above means that Arbitron has met or exceeded their sample target. A DDI below 100 means that Arbitron has missed their sample target.
The compliance rate tells you how many people are following Arbitron’s rules for inclusion in the estimates. First, we take the average daily in-tab persons and subtract out anyone who was incapable of compliance – these are panelists who were known to be away from home, as well as those experiencing technical difficulties that prevented them from being compliant. Once we’ve calculated our compliance-capable persons, we compare it to the average daily in-tab. If 1,900 panelists were compliance-capable on an average day and 1,700 people were in-tab on an average day, the compliance rate would be 89.5%.
Turnover occurs when a panelist decides to stop participating, when a panelist is removed from the panel due to non-compliance, or when a panelist has been in the panel for two years. Panel turnover measures help us understand how many panelists dropped out or were removed from the panel over a given four-week survey period. The calculation is made by taking the number of people that were in the panel the first day of the survey but not the last and dividing it by the number of people who were in the panel the first day. Typical panel turnover is between 5% and 10%. Turnover is important because a heavy listener coming in or going out of the panel can have a noticeable effect on your estimates.
Sample Performance Indicator (SPI)
Similar to response rate, the Sample Performance Indicator is a way to measure the performance of a panel. Arbitron draws two random samples – one of Basic Households and one of Alternate Households. They begin recruiting within the Basic Households sample. If they are unable to get agreement from one of these households, they will then recruit from the Alternate Households sample. The SPI is the percentage of in-tab panelists that come from the Basic Households sample group.
Your station’s ratings are impacted by more than just what you and your competitors do on the air. Arbitron’s sample is the foundation. We hope this has armed you with the knowledge you need to determine how sampling changes might have impacted your ratings.
-Anne Doyle, Production Manager