A study of the variability of labour productivity in building trades

This study focuses on certain aspects of labour productivity in the masonry, wall plastering, tiling and floor screeding trades. Aspects such as the variability in productivity values, the incidence of delays and the effect of these delays on productivity are examined for each of these trades. A method of monitoring labour productivity known as the daily visit method was used to collect productivity data on the four trades. This method is less labour intensive and simpler to use than the activity sampling and continuous observation methods. Delays of length less than 15 minutes were ignored in the daily visit method and it was found that this lead to productivity values which were consistently about 21% lower than the true values. However, it was also found that delays of 15 minutes and less in duration did not significantly affect productivity in any of the four trades and that if these delays are ignored, productivity values obtained from the daily visit method approximated to the true values. Daily productivity measured in terms of the paid hours per day had a coefficient of variation of 34% for the masonry trade. Corresponding values for the wall plastering, tiling and flooring trades were found to be 28%, 20% and 9% respectively. For each of the trades it was found that these values were all higher than the coefficient of variation values of productivity measured in terms of the daily time spent at work. The shape of the probability distribution of productivity values for all the trades was approximately normal. Two types of delay were defined. Type 1 delays were unavoidable and due primarily to inclement weather conditions. These delays accounted for 42% of all the delays experienced by the masonry trade but were absent in the finishing trades. Of the four trades studied, the masonry trade was the only trade affected by material delays. Approximately 35% of all the delays experienced by the masonry trade were due to a lack of materials. At least 90% of all the delays experienced by the finishing trades were due to sequencing problems involving the work of other trades and to a lack of clear and adequate working instructions. These two causes of delays accounted for only 15% of all the delays experienced by the masonry trade. An intervening delay was defined to be a delay which occurred once work had commenced. About 66% of all the delays experienced by the masonry trade were intervening delays. At least 85% of all the delays experienced by the finishing trades were intervening delays. Intervening delays were found to result in a loss in productivity. A theory called The Intervening Variable Theory which is based upon the psychophysical laws of Fechner and Stevens was developed to explain the relationship between productivity and intervening delays. Using this theory, the effect of intervening delays on productivity was modelled by simple power curves. The differing effects of an intervening delay of a given length on productivity for the trades was attributed to the manner in which the trade was organised and the peculiar operational characteristics of each of the trades. The ability of a trade to buffer the effect of an intervening delay was defined as its Storage Capacity. The power curve models allowed for the loss in storage due to intervening delays to be evaluated for each of the trades.