Edited sentence: A study would then require a reliable measurement of both the extent of individual exposure and of the frequency of lung cancers in different groups of non-smokers.Because it is impossible to find persons never exposed to ETS, the only real possibility is to observe if the frequency of lung cancer is higher or lower in non-smokers that are more or less exposed.Because there are many other proven risks for lung cancer, a study also must find whether individual lung cancers in non-smokers might be linked to other risks and not to ETS.Confounders - According to summaries conducted by groups that have found elevated risks for ETS, the average of all studies on lung cancer and passive smoke published up to May 2006 (about 75) claims a risk elevation of some 20 per cent.ETS exposure has not been measured as it would be to expensive.Instead 60-to-70-year-old non-smokers have been asked to recall what their personal exposure to ETS might have been during their lifetimes.Measurements - Because lung cancer is a disease that develops slowly and manifests itself for the most part at an advanced age, the exposure to ETS needs to be measured over the lifetime of non-smokers.People or their next of kin were asked via questionnaires to recall how many cigarettes, cigars or pipes had been smoked in their presence since early childhood.Self-declared non-smokers all with lung cancer and exposed to ETS have been compared to self-declared non-smokers without lung cancer, the latter also exposed to ETS because it is impossible to find never exposed people.Such a relatively low risk is insignificant because the studies have not accounted for a whole series of other known risks of lung cancer, and biases that are inevitably present.Such presumption also implies the reasoning that remembering a 20 per cent excess exposure had been responsible for all the lung cancers of the non-smokers with the disease, while non-smokers who remember only a little less exposure remain totally immune from lung cancer.The Methodology - The overwhelming majority of ETS studies does not define risk on the necessary basis of higher or lower frequency of cancer in function of higher or lower exposures to ETS.The possible risk of ETS for lung cancer or cardiovascular disease could be determined if the frequency of lung cancer or cardiovascular disease is greater in non-smokers exposed to ETS.These persons recalls were recorded by the studies as precise numbers devoid of error and uncertainty