Data Trending

Repeated measurements over time are used to track changing trends. When measurements are based upon sample surveys over time, data at different points in time may be different due to different reasons. The most obvious reason is that the underlying phenomenon has changed. An alternate reason is that the changes are due to sampling error, even though the underlying phenomenon has not changed at all. It is not an easy task to sought out the one from the other.

Without amassing the vast array of statistical tools that can be brought to bear on this issue, we shall look at a couple of variables across time in the Los Medios y Mercados de Latinoamérica study.

First of all, we look at the percent of persons using radio by daypart across three years. These numbers are shown in Table 1. In most cases, the annual numbers are within 1 or 2 percentage points of each other. We can fairly conclude that these numbers are stable over time. If anything, we would wonder that such stable behavior needed to be tracked as frequently as we have.

Table 1. Percent of Persons Using Radio by Daypart (1994-1995-1996)

(Source: Los Medios y Mercados de Latinoamérica 1994, 1995, 1996)

As another example, we consider the consumer confidence index. The definition is given elsewhere. The numbers are shown in Table 2. Here we see that there are some considerable fluctuations from year to year. From our first example, which is based upon the same sample of respondents, we know that the data can be relatively stable when the underlying phenomenon is unchanging. Therefore, we would now be inclined to believe that the consumer confidence indices may be reflecting real changes in perceptions and attitudes. In fact, we observe that the largest decreases in consumer confidence occurred in the two countries that were hurt most severely by the Mexican peso crisis --- Argentina and Mexico.

The magnitude of the changes from year to year would suggest that variables such as consumer confidence indices should be measured even more frequently (such as on a monthly basis) in order to react in a timely manner.

Table 2. Consumer Confidence Index Versus One Year Ago by Country, 1994-1995-1996

(Source: Los Medios y Mercados de Latinoamérica 1994, 1995, 1996)

(posted by Roland Soong, June 20, 1997)

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