Arbitron pledges less bounce.
Under existing rules, almost 20% of a market’s PPM panel could turn over during a three-month period. That often causes wide ratings swings from month to month as new sample is ferried in to replace panelists who had reached the expiration point of their tenure. But changing those rules should mean less ratings bounce.
Beginning with the November survey, Arbitron will implement new sample expiration rules intended to smooth out the turnover. The rules are expected to be applied in all PPM markets by March 2011. The current rule calls for initially selected basic sample to expire between 18 and 36 months after the panel build and for basic sample selected after the initial build period to expire at exactly 24 months. “This was creating a spike in turnover right around 24 months after a panel was filled,” Arbitron’s Beth Webb says. The increase in turnover has been exacerbated by improvements in sample compliance. “As we’ve improved compliance, people are staying in the panel longer,” Webb says, with many remaining for the full 24 months after the sample build, resulting in a large number being replaced simultaneously. “It creates a lot of turnover in the panel all at once, which creates increased variance in the estimates,” Webb says. “We wanted to reduce the impact by smoothing over the turnover.”
Under the new rule, all initially selected sample expires between 18 and 30 months while the maximum panel tenure after the initial build period is set at 24 months. “Once we’ve spread out that turnover from the initially selected sample, ongoing sample will expire between 18 and 24 months, so we’ll continue to smooth out the turnover,” Webb says. “You won’t get a three-month period where there’s a huge turnover in the panel. This will help the stability of the estimates in that period.”