THE COMMON ROOM
Page 66
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By George Wilmot
Lecturer in Transport Studies. tiniversity of London
The Use and Abuse of Statistics in Road Transport-2, Averages
IN 1962 the Ministry of Transport completed a largescale survey of goods transport by road. Last year the results of this survey were published in two parts in a number of statistical tables. The first part dealt with information about the size, structure and carrying capacity of the goods vehicle. Later the work performed by road transport was broken down into commodities per licence type, which was published as Part II.
The survey gives a welter of statistics in which it is all too easy to become bewildered by the sheer volume of figures. The big problem is how to summarize the information and use it for the particular problem with which one is faced.
Suppose one was seeking information about the carrying capacity of C-licensed vehicles. The most relevant table is on page 27 of the Statistical Table No. 2 of Part I of the Survey:
The table shows, for example, that more than f m. C-licensed vehicles have a capacity of up to one ton, while a mere 1,400 have a capacity of more than 16 tons.
But this information seems to be lengthy if, for example, a short, succinct comment had to be presented on this aspect. The obvious instinct is to cut down this data and reduce it to an average figure. Immediately there is a snag as there are three recognizable ways of expressing an average, each of which could be employed in this example. Two of these averages are termed the " mode " and the 'median ". A third average—the "arithmetic mean "will be discussed next week.
Mode and Median
The mode is the average which expresses the most frequently occurring typein this example, the most frequently occurring capacity of C-licensed vehicle. The table shows clearly that up to one ton is the most typical carrying capacity.
The median is the middle size of C-licensed vehicle obtained by arranging all the 1.226,400 vehicles in order of capacity and finding the middle one. Thus, half the vehicles will be larger and half the vehicles will be smaller than it. In this example it will be the 613.200th vehicle, which will be in the middle once they have been arranged in size. The table does not give sufficient detail to tell its exact carrying capacity but it will be less than one ton weight as more than half are in that category. Thus, the mode and the median express the great preponderance of light ('-licensed vehicles.