Ecological Momentary Assessment (EMA) was formally defined in 1994 (Stone et al., 1994; Stone, Shiffman, & DeVries, 1998) to expand momentary sampling methods from experiences (as per the name) to experiences, behaviors, and physiological measurements. All EMA studies contain three qualities. The first is that measurements are made in the environments that people typically inhabit to ensure ecological validity. This is a characteristic of many of the above-mentioned methods (e.g., ambulatory monitoring of cardiovascular function, the ESM, and circadian rhythm work).
The second characteristic is the measurement of momentary phenomena to eliminate or at least greatly reduce biases associated with recall. Thus, participants are asked to report about their experiences, behaviors, and environment, or to take physiological measures (e.g., activate a blood pressure monitor, although this can also be accomplished automatically via programming those devices) at the moment of the signal. In fact, however, many investigators have made informed decisions about the period of recall considered acceptable for their research goals and have thus asked participants to report about a period prior to the signal (e.g., 5 minutes, 30 minutes, etc.). Care must be taken when extending the assessment for the moment, given the possibility that recall bias may contaminate even seemingly brief recall periods.
The third EMA quality is that many momentary reports are taken from each participant, yielding a within-person design. Multiple observations are important in three ways:
1. They can be averaged to yield a measure that represents the level of experience (or behavior, environment, etc.) for an individual. Given the large number of observations contributing to an average, this should be a relatively stable estimate. Researchers can also estimate the measures' variability by computing a dispersion statistic like a standard deviation, which may be useful for assessing individual differences.
2. Multiple observations allow a detailed examination of the variable over time, so that, for instance, cyclicity of the variable (within-day, over days) can be examined.
3. Many observations of the variable of interest may be associated with other momentary variables yielding knowledge about within-person associations (also known as time-varying covari-ates). One might, for instance, be interested in the level of negative affect according to whether or not the individual was with other people (and who those people were). However, thoughtful selection of the sampling strategy used to collect momentary data is essential to address study hypotheses, because the study conclusions depend on the sampling schemes.
Many review chapters have been published on diary methods and real-time data capture, which the reader may consult to appreciate the versatility and broad scope of the application of EMA and related methods (Affleck, Zautra, Tennen, & Armeli, 1999; Bolger, Davis, & Rafaeli, 2003; Delespaul, 1995; DeVries, 1992; Eckenrode & Bolger, 1995; Reis & Gable, 2000; Stone, Kessler, & Haythornthwaite, 1991; Stone, Shiffman, & Atienza, in press; Tennen & Affleck, 2002). Special issues of major journals (e.g., Health Psychology, Journal of Nervous and Mental Disorders, Annals of Behavioral Medicine) have published special sections or issues on the topic.
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