  Obtaining an accurate picture of the current state of the economy is
particularly important to central banks and finance ministries, and of
epidemics to health ministries. There is increasing interest in the use of
search engine data to provide such 'nowcasts' of social and economic
indicators. However, people may search for a phrase because they independently
want the information, or they may search simply because many others are
searching for it. We consider the effect of the motivation for searching on the
accuracy of forecasts made using search engine data of contemporaneous social
and economic indicators. We illustrate the implications for forecasting
accuracy using four episodes in which Google Flu Trends data gave accurate
predictions of actual flu cases, and four in which the search data
over-predicted considerably. Using a standard statistical methodology, the Bass
diffusion model, we show that the independent search for information motive was
much stronger in the cases of accurate prediction than in the inaccurate ones.
Social influence, the fact that people may search for a phrase simply because
many others are, was much stronger in the inaccurate compared to the accurate
cases. Search engine data may therefore be an unreliable predictor of
contemporaneous indicators when social influence on the decision to search is
strong.
