4 - Co-occurrence, tipping in, and bridging

Summary

We had not peas nor strawberries here till the 8th day of this month. On the same day I heard the first whip-poor-will whistle. Swallows and martins appeared here on the 21st of April. When did they appear with you and when had you peas, strawberries, and whip-poor-wills in Virginia? Take notice hereafter whether the whip-poor-wills always come with the strawberries and peas.

Letter from Thomas Jefferson to Maria Jefferson (June 13, 1790)

For all the credit given Jefferson as statesman, architect, and inventor, he is rarely noted in the annals of empirical science. Believing that matrices of cooccurring events would yield an understanding of the laws that governed nature, Jefferson searched for instances of combination and of the interdependence of phenomena. He maintained diaries of astronomical events, of rainfall, and of the first appearance and subsequent co-occurrence of about forty types of vegetables, using these data to speculate about “natural laws,” and, more pragmatically, to make decisions about ploughing, planting, and crop rotation.

Co-occurrence is an interesting problem for this investigation since a greater than chance probability of co-occurrence of two or more types of cultural suppliers in a large city provides an indication of either (1) organizational interdependencies, or (2) that critical masses involving substantial numbers of people share a taste for at least two types of culture.