With slight modification the statement “Accidents do not distribute themselves by chance but . . . happen frequently to some men and infrequently to others as a logical result of a combination of circumstances” has become a generalization. This statement embodies the principle of accident proneness. Accident proneness is a hypothesis about human behaviour which says that accident behaviour is not a random phenomenon. Rather, it is a consistent characteristic which can be predicted. The accident proneness concept does not hold that accidents are caused strictly by chance. One can represent this concept with the simple equation
AT = ae + ap
Where AT = total number of accidents
ae = accidents caused by chance factors
ap = accidents attributable to personal characteristics
Those who believe in accident proneness believe that ap is an important source of accidents.
Mintz and Blum, who have critically evaluated the literature in the field, have come to the conclusion that accident proneness is overrated (1948). A correct interpretation demands that the second half of the statement be worded as follows: Accidents happen frequently to some men, that is, some men have more accidents than would be expected by chance, and infrequently to others, that is, other men have fewer accidents than would be expected by chance.
In accordance with chance expectancy, some men will have no accidents some will have one accident, some two accidents, and some three or more accidents. In other words, chance will result in a distribution of accidents, and it is wrong to assume that in accordance with chance expectancy all men should have equal numbers of accidents.
To establish or support the principle of accident proneness, three methods can be used. In one method, the distribution of the total number of accidents in a population is compared with the distribution that would be expected if only chance factors operated. A comparison of these two distributions makes it possible to determine whether accidents happened more frequently to some men than would be expected by chance.
The second method is to study the individuals and the number of accidents they have in two successive periods. A tendency for these people to have similar numbers of accidents can be regarded as evidence in favour of accident proneness. The third method is to compute the correlation coefficient of the accident records of a group for two periods.
Mintz and Blum (1949) found a considerable amount of careless reporting, illogical reasoning, and unfamiliarity with the statistical theory underlying accident proneness. Their analysis shows that 60 to 80 percent of the accidents reported appear to be attributable to unpredictable factors and the remaining 20 to 40 percent to the component of accident liability which includes both personal characteristics and environmental conditions contributing to accident records. Thus accident proneness does not seem to be the major factor in understanding or attacking the problem of accidents.
The most frequently offered evidence for the existence of accident proneness is the fact that a small percentage of the population has a large percentage of the total number of accidents. Such statements, by themselves, do not substantiate accident proneness. The obvious error in them is made clear by the following hypothetical situation: Two hundred employees have 100 accidents. If every employee who has an accident has only one, there will be the opportunity for only 100 employees to have an accident record, and yet this leads to the conclusion that 50 percent of the employees have 100 percent of the accidents.
This situation is extremely unreal, for there is no reason to believe that each employee should have one accident. According to chance, approximately 121 in a population of 200 should have no accidents, 61 people should have one accident, 15 people should have two, and 3 people should have three accidents. On this basis, the expectancy is that 9 percent of the population will have 39 percent of the accidents and that 39.5 percent of the population will have 100 percent of the accidents. This distribution is due solely to chance expectancy; there is no evidence of accident proneness.
If a deck of cards is dealt to four people it sometimes happens that one of them will get six, seven, or more hearts instead of three or four. Such a distribution is normally attributed to chance. Statements to the effect that 10 percent of a population have 30 percent of the accidents or that 25 percent have 75 percent of them must be regarded as inconclusive unless the total population and the total number of accidents are known. Only when we have these figures can we establish the extent to which the distribution of accidents is due to chance expectancy and the extent to which other factors such as accident proneness enter.
An early study which is often referred to and which finds evidence for accident proneness was conducted by Greenwood and Woods (1919). The data in this study are rather complete, which is more than can be said about many of the more recent studies. These authors develop the statistical formula based upon a theory of accident proneness and this must be recognized as a contribution. Table 18.1 presents some of the data from this study.
If the principle of accident proneness is to be upheld, more people should have no accidents than would be predicted by chance. In three of the five groups this holds true, but in the other two it does not. In the three groups where it does hold true, the percentages are 6, 3, and 10 percent.
These percentages are to be attributed to accident proneness in certain people, but they must also be attributed to other causative factors such as length of employment, job hazard, training, etc. In any event, accident proneness as a cause for accidents is apparently not so formidable as some authorities have made it. Although Greenwood and Woods do establish the principle of accident proneness, they do not indicate the degree to which it is responsible for accidents; many others who have done research in this field imply that the extent of accident proneness has been exaggerated.
Another erroneous use of accident proneness is seen in the arbitrary classification of people who have more than the average number of accidents as accident prone. Such a method was used in the Cleveland Railway study, and at least one textbook defines accident proneness as being present in people who have two or three times as many accidents as the average person has.
In Table 18.1 the average number of accidents per person is approximately 0.5; hence in an arbitrary classification a person having one or two accidents would be accident prone. This is not necessarily true. The hypothetical situation shows clearly that some people may have two or three accidents solely on the basis of chance expectancy. This critical attitude toward the concept of accident proneness is in part supported by Cobb (1940) and Johnson (1946). More work along these lines should be encouraged.
Arbous and Kerrich have also made an exhaustive search of the literature on the topic of accident statistics and accident proneness (1951). Like Mintz and Blum, they believe that the knowledge of this concept has hardly proceeded further than the early studies of Greenwood and in some respects the topic has actually suffered a reverse because of the misunderstanding. Their article is an exceedingly well- written and readily understandable one and should be a standard reference for the more serious student in this area.
They raise such important points as the correlation between minor accidents for two successive periods, the correlation between major accidents for two successive periods, the correlation between minor and major accidents, and the correlation between different types of accidents.
They conclude with the comment: “‘Accident-proneness Percy’ is a figment of the imagination resulting from wishful thinking.” This points to a view of accident prevention since it means that an individual’s liability or proneness to accidents (when such a thing exists) in one set of circumstances will give little indication of proneness in another The cogent remark that might best summarize the point of view of Arbous and Kerrich is as follows: “This does not mean that accident proneness does not exist but that so far we have not succeeded in defining it, assessing its dimensions and constituent elements, nor evolved a technique for putting it to practical use.”