CROSS SECTIONAL ANALYSES
34. In terms of the relationship between gun ownership and homicide, cross sectional analyses, comparing one country with another or comparing groups of countries, create enormous problems because of the huge number of variables that may exist. The simpler of these variables are the different methods of defining and counting the number of firearms. Only those firearms which have been declared to the authorities can be counted. In some countries, licenses are required for almost all classes of firearms, including antiques and air weapons. In other countries, the term antique is extended to cover items considered to be subject to license elsewhere, and many classes of shotgun and rifle are not controlled so are not countable.
35. Homicide statistics too vary widely. In some developing countries, the statistics are known to be far from complete. Figures for crimes labelled as homicide in various countries are simply not comparable. Since 1967, homicide figures for England and Wales have been adjusted to exclude any cases which do not result in conviction, or where the person is not prosecuted on grounds of self defence or otherwise. This reduces the apparent number of homicides by between 13 per cent and 15 per cent. The adjustment is made only in respect of figures shown in one part of the Annual Criminal Statistics. In another part relating to the use of firearms, no adjustment is made. A table of the number of homicides in which firearms were used in England and Wales will therefore differ according to which section of the annual statistics was used as its base. Similarly in statistics relating to the use of firearms, a homicide will be recorded where the firearm was used as a blunt instrument, but in the specific homicide statistics, that case will be shown under "blunt instrument".
36. Many countries, including the United States, do not adjust their statistics down in that way and their figures include cases of self defence, killings by police and justifiable homicides. In Portugal, cases in which the cause of death is unknown are included in the homicide figures, inflating the apparent homicide rate very considerably.
37. Causing death by dangerous driving is not classed as homicide in England and Wales, but is classified as homicide in some countries. Over 200 such cases occur in England and Wales each year.
38. In France, Switzerland and several other countries, attempts and completed homicides are treated as a single statistical unit and can be separated out only by special enquiry.
39. The variables created by the factors listed above could be either eliminated or controlled-for in a thorough study. Other variables are much more difficult to eliminate or balance. Social, ethnic, historical and geographical factors have been shown to be extremely important, and police efficiency, arrest rates and sentencing policies which differ from country to country may be important in some classes of homicide, but are less important in others.