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Mill’s Methods. Method of Agreement. ABCD precedes abcd AEFG precedes aefg A is the cause of a.
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Method of Agreement ABCD precedes abcd AEFG precedes aefg A is the cause of a
A 1994 study by Dr. Stephen Moses, published in the International Journal of Epidemiology, showed that around the world men who were not circumcised were three to four times likelier to be infected with H.I.V. than circumcised men. One hypothesis suggested was that the virus may be transmitted through tears in the foreskin during intercourse. A study reported in Scientific American in March of 1996 claimed to show that "only one factor" seemed to correlate with susceptibility to H.I.V. infection in Africa: lack of circumcision.
Method of Agreement • This method works by looking for an agreement between A (cause) and a (effect) in varied situations. • In this case, the “varied situations” are all the different men around the world who are susceptible to H.I.V. infection. • The agreement between all these differences is that all these different men are not cicumcised.
Method of Agreement ABCD precedes abcd AEFG precedes aefg A is the cause of a Man 1 is not circumcised and is susceptible to HIV infection Man 2 is not circumcised and is susceptible to HIV infection ………. Man n is not circumcised and is susceptible to HIV infection ----------------------------- "only one factor" seemed to correlate with susceptibility to H.I.V. infection in Africa: lack of circumcision.
Method of Difference ABCD precedes abcd BCD precedes bcd A is the cause of a
Inflammation-swelling, redness, and pain-plays a key role in rheumatoid arthritis and in the process that leads to diabetes. Can the gene that causes inflammation be identified? Dr. Donald N. Cook, a pathologist at the University of North Carolina at Chapel Hill, was able to do so using what are called "knockout mice." Like humans, mice suffer inflammation as a result of many infections, and like humans they possess the gene MIP-1 alpha, suspected of producing the protein that begins the process of inflammation. Dr. Cook and his team bred mice that lacked the gene MIP-1 alpha, and then infected those mice, and a control group of normal mice, with the influenza virus and the coxsackie virus (which can cause heart damage in children and young adults). In response to the infection, all the normal mice developed extreme inflammation, with swelling and redness. But the mice lacking the MIP-1 alpha gene had only slight inflammation. The experiment, said Dr. Cook, proves that the MIP-1 alpha gene promotes inflammation in response to virus infection. This finding, he suggested, could lead to the development of drugs that would allow the body to fight viral infections without the damaging effects of inflammation.
Method of Difference • Tip: • The Method of Difference usually utilizes a control group (a sample that is just like the typical sample/population, except that it is missing one feature) • In this case, the knock-out mice lacked the gene MIP-1 alpha
Method of Difference ABCD precedes abcd BCD precedes bcd A is the cause of a Mice with the MIP-1 alpha gene that were infected with the coxsackie virus developed inflammation. Mice without the MIP-1 alpha gene (the knockout mice) did not develop inflammation when infected the coxsackie virus. Therefore, MIP-1 alpha gene promotes inflammation in response to virus infection.
Joint Method of Agreement and Difference ABC—xyz ABC—xyz ADE—xtw BC—yz Therefore A is the effect or the cause of x
Joint Method of Agreement and Difference • The use of this joint method is what often underlies what are called “double-arm” or crossover experiments. This is quite common, e.g., in medical experiments. • One group receives a new treatment while another group does not, and then in a second phase those who did not receive the treatment, do so, and those who did, have it withdrawn.
Pasteur ... carried out at least one spectacular experiment having to do with the effect of temperature on susceptibility to infection. Puzzled by the fact that hens were refractory to anthrax, he had wondered whether this might not be explained by their body temperature, which is higher than that of animals susceptible to this disease. To test his hypothesis, he inoculated hens with anthrax bacilli and placed them in a cold bath to lower their body temperature. Animals so treated died the next day, showing numerous bacilli in their blood and organs. Another hen, similarly infected and maintained in the cold bath until the disease was in full progress, was then taken out of the water, dried, wrapped, and placed under conditions that allowed rapid return to normal body temperature. Mirabile dictu, this hen made a complete recovery. Thus, a mere fall of a few degrees in body temperature was sufficient to render birds almost as receptive to anthrax as were rabbits or guinea pigs.
Joint Method of Agreement and Difference Agreement Rabbits have a normally low temperature and are susceptible to anthrax. Guinea pigs have a normally low temperature and are susceptible to anthrax. Hens whose body temperature has been lowered (by cold bath) to a below normal temperature are susceptible to anthrax. Therefore, “a mere fall of a few degrees in body temperature was sufficient to render birds almost as receptive to anthrax as were rabbits or guinea pigs” Difference Hens whose body temperature has been lowered to a below normal temperature and contract anthrax will die. Hens whose body temperature has been lowered to a below normal temperature and contract anthrax and are then warmed to their normal temperature recover from anthrax. Therefore, “a mere fall of a few degrees in body temperature was sufficient to render birds almost as receptive to anthrax as were rabbits or guinea pigs”
Method of Residues ABC—xyz B is the cause of y C is the cause of z A is the cause of x
Method of Residues • Some tips: • Typically used when we remove from the phenomenon the casual features that we already know, and what remains (the residue) is (probably) causally related. • Resembles a process of elimination.
In H. Davies' experiments on the decomposition of water by galvanism, it was found that besides the two components of water, oxygen and hydrogen, an acid and an alkali were developed at opposite poles of the machine. Since the theory of the analysis of water did not give reason to expect these products, their presence constituted a problem. Some chemists thought that electricity had the power of producing these substances of itself. Davies conjectured that there might be some hidden cause for this part of the effect-the glass might suffer decomposition, or some foreign matter might be in the water. He then proceeded to investigate whether or not the diminution or total elimination of possible causes would change or eliminate the effect in question. Substituting gold vessels for glass ones, he found no change in the effect and concluded that glass was not the cause. Using distilled water, he found a decrease in the quantity of acid and alkali involved, yet enough remained to show that the cause was still in operation. He inferred that impurity of the water was not the sole cause, but was a concurrent cause. He then suspected that perspiration from the hands might be the cause, as it would contain salt which would decompose into acid and alkali under electricity. By avoiding such contact, he reduced the quantity of the effect still further, till only slight traces remained. These might be due to some impurity of the atmosphere decomposed by the electricity. An experiment determined this. The machine was put under an exhaust receiver and when it was thus secured from atmospheric influences, no acid or alkali was produced.
In H. Davies' experiments on the decomposition of water by galvanism, it was found that besides the two components of water, oxygen and hydrogen, an acid and an alkali were developed at opposite poles of the machine. Since the theory of the analysis of water did not give reason to expect these products, their presence constituted a problem. Some chemists thought that electricity had the power of producing these substances of itself. Davies conjectured that there might be some hidden cause for this part of the effect-the glass might suffer decomposition, or some foreign matter might be in the water. He then proceeded to investigate whether or not the diminution or total elimination of possible causes would change or eliminate the effect in question. Substituting gold vessels for glass ones, he found no change in the effect and concluded that glass was not the cause. Using distilled water, he found a decrease in the quantity of acid and alkali involved, yet enough remained to show that the cause was still in operation. He inferred that impurity of the water was not the sole cause, but was a concurrent cause. He then suspected that perspiration from the hands might be the cause, as it would contain salt which would decompose into acid and alkali under electricity. By avoiding such contact, he reduced the quantity of the effect still further, till only slight traces remained. These might be due to some impurity of the atmosphere decomposed by the electricity. An experiment determined this. The machine was put under an exhaust receiver and when it was thus secured from atmospheric influences, no acid or alkali was produced.
Method of Residues ABC—xyz B is the cause of y C is the cause of z A is the cause of x Galvanized water produces: hydrogen, oxygen and an alkali and acid. The alkali and acid were unexpected. What was the cause? Was it the water? Was it the electricity? Was it the glass vessel the water was in? It was then hypothesized that the acid and alkali were caused by impurities, impurities in the water, on the hands of the experimenters, and in the atmosphere. Experiments bore this out…
Method • The Method of Concomitant Variation ABC—abc A+BC—a+bc A and a are connected, but not necessarily the cause
Careful studies have been made of the incidence of leukemia in the survivors of the atomic bombs burst over Hiroshima and Nagasaki. These survivors received exposures ranging from a few roentgens to 1000 roentgens or more. They are divided into four groups.... The first group, A, consists of the estimated 1,870 survivors who were within 1 kilometer of the hypocenter (the point on the surface of the earth directly below the bomb when it exploded). There were very few survivors in this zone, and they received a large amount of radiation. The second group, B, consists of the 13,730 survivors between 1.0 and 1.5 kilometers from the hypocenter; the third, C, of the 23,060 between 1.5 and 2.0 kilometers; and the fourth, D, of the 156,400 over 2.0 kilometers from the hypocenter. The survivors of zones A, B, and C have been dying of leukemia during the period of careful study, the eight years from 1948 to 1955, at an average rate of about 9 per year.... Many more cases of leukemia occurred in the 15,600 survivors of zones A and B than in the 156,400 survivors of zone D, who received much less radiation. There is no doubt that the increased incidence is to be attributed to the exposure to radiation. ... The survivors of zone A received an estimated average of 650 roentgens; those of zone B, 250; those of zone C, 25; and those of zone D, 2.5.... To within the reliability of the numbers, the incidence of leukemia in the three populations A, B, and C is proportional to the estimated dose of radiation, even for class C, in which the estimated dose is only 25 roentgens.
The Method of Concomitant Variation • Look out for cases where there are fluctuating levels in the cause/effect corresponding to fluctuating levels in the effect/cause. • In this case, there is a correlation between amount of radiation a group is exposed to and the incidence of leukemia in that group.
The Method of Concomitant Variation ABC—abc A+BC—a+bc A and a are connected, but not necessarily the cause Exposed to 650 R results in high incidences of leukemia. Exposure to 250 R, a little less… Exposure to 25 R, still less… ------------------------------ Therefore: To within the reliability of the numbers, the incidence of leukemia in the three populations A, B, and C is proportional to the estimated dose of radiation, even for class C, in which the estimated dose is only 25 roentgens.
