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7.22.            Reference period  Additionally, because of memory effects, the period of reference to be used, regarding both travel/trips and the expenditure associated with them, must be very short. The present Guide considers it good practice to use a month as a reference period in order to reduce telescopic errors (improper date assigned to trips) and recalling errors (improper characteristics and expenditure) (see box 7.2). As a consequence, because most persons might not have made any trip during such a short period of reference, the selected sample should be sufficiently large so as to collect enough valid information.[1] A solution could be to ask about travel in the last x months and then deal with the memory effect by weighting up information relating to older travel. 




Box 7.2

Research on the effect of expanding the period of reference for reporting tourism trips in Spain[2]

Research carried out by the Instituto de Estudios Turísticos (IET) of Spain confirms that relying on a respondent’s memory for information covered by a domestic and outbound tourism survey can cause two distinct types of errors entirely unrelated to sampling, and that have often opposite signs: 

  • Telescopic error: the actual date of an event (an expense, a trip) is unconsciously and erroneously moved up to a more recent date;
  • Pure memory effect: an event that, in fact, occurred during the observation period is simply omitted, because, subjectively, by the time of the interview, it seems to have occurred long before. 

IET developed a complex observation methodology using three overlapping samples, in which respondents were asked about trips taken three months, two months and one month before being contacted. That study clearly demonstrated that the longer the delay in contacting respondents after the observation period, the fewer trips they report. For example, three statistically validated measurements for the number of trips taken in June 2006 by residents of Spain are as follows:

  • 12,991,044 if observed in July
  • 12,745,211 if observed in August
  • 12,247,920 if observed in September

 Source: Guardia, T. and Garcia, S. (2008)

 7.23.            Issues to consider  For the process of selecting households, individuals and travel/trips, various possibilities are conceivable and each of has its advantages and disadvantages.  Regarding the selection of households: