… is from page 347 of the 2014 collection, The Market and Other Orders (Bruce Caldwell, ed.), of some of F.A. Hayek’s essays on spontaneous-ordering forces; this particular quotation is from Hayek’s important January 1970 lecture at the University of Salzburg, “The Errors of Constructivism“:
However greatly our theories and techniques of investigation [such as econometrics] assist us to interpret the observed facts, they give little help in ascertaining all those particulars which enter into the determination of the complex patterns, and which we would have to know to achieve complete explanation, or precise predictions.