The impact of portfolio aggregation on day-of-the-week effect: Evidence from Finland

This paper revisits the day-of-the-week anomaly discussion from a portfolio aggregation point of view. Using different levels of portfolio aggregation: market, industry and company levels, it is possible to partly trace the aggregation level of the effect of the factors driving the day-of-the-week anomaly. The effect of portfolio aggregation is measured using a conditional modeling approach. Overall, the results indicate more pronounced day-of-the-week structures in the conditional volatility than in the mean returns and considerably more day-of-the-week structures during the post-euro period. For this period the results indicate that the day-of-the-week effect in the mean is partly a common Finnish market characteristic whereas the day-of-the-week effect in the volatility is found on the industry level of portfolio aggregation.