Autocorrelation and partial price adjustment

•We separate stock return autocorrelation into spurious and genuine components.•We use sixteen years of NYSE intraday transaction data.•We study both individual stock and portfolio return autocorrelation.•We find that partial price adjustment is a major source of autocorrelation.

Stock return autocorrelation contains spurious components—the nonsynchronous trading effect (NT) and bid–ask bounce (BAB)—and genuine components—partial price adjustment (PPA) and time-varying risk premia (TVRP). We identify a portion that can unambiguously be attributed to PPA, using three key ideas: theoretically signing and/or bounding the components; computing returns over disjoint subperiods separated by a trade to eliminate NT and greatly reduce BAB; and dividing the data period into disjoint subperiods to obtain independence for statistical power. Analyzing daily individual and portfolio return autocorrelations in sixteen years of NYSE intraday transaction data, we find compelling evidence that PPA is a major source of the autocorrelation.