"Of the extrapolation of the malfunction", or "this sample is not representative"

Since they will know, the pharmacist Lilly and the Association for the Investigation of Sexual Malfunctions in Primary health care (AIDS-AP) have made a study public on sexual life and sexual ailments of the Spanish males. In the press release (I have not found the study in the web), they refer all the time to the Spanish men, on having presented the results, as if the survey on which the study is based was representative of the Spanish males, of those of 18 to 95 years, in this case. One of the information most spread by the newspapers is that of 42 % "of the Spanish men" that he endures of erectile malfunction.
In the press release it is not said to us much any more how the sample is obtained. There is mentioned this criterion, which 720 centers have informed for the whole Spain and which has been selected to males from 18 to 95 years. It is not said to us, for example, when the field work has taken place. Yes one mentions that 58 % had 50 years or more.
In fact, the survey cannot be representative. The sample is obtained of the males who come to a primary health care consultation for any motive, but not all the males come to the primary health care doctor with the same frequency. For example, it goes away more to the doctor as the age increases, as it can be verified easily with the information of the last National Survey of Health, of the INE, corresponding to 2006. Then, it had come to a medical consultation in the last four weeks 61 % of the 75-year-old males or more, but only 22 % of the from 18 to 24. Also, independently of the age, which are healthier will go less to the doctor that those who are more rotten.
In spite of little information that they offer, they are enough two of them to realize that the sample is not representative.
On one hand, if out, the percentage of 50-year-old individuals or more it would be 38 %, as it can be calculated by the information of estimations of the current population (on January 1, 2009) of the INE, and not of 58 % as they affirm. Since it was necessary to wait, bearing in mind the rules of assistance to the doctor for ages, his sample is too "old".
On the other hand, probably for the same reason (that of a too high middle age) or for the reason of that those who more come are the least healthy, another fact also suggests that the sample is not representative. They say in the press release that 39 % was enduring arterial hypertension. Nevertheless, in the National Survey of Health already said (ENS), of three years ago, the males (16 years or more) that were declaring to have arterial hypertension were 17 % or 19 %, according to the indicator that is used. Many less, in any case. (1)
Take this study (good, this press release) as an example of the problematic thing of extrapolating to the set of the population the obtained information of the public who comes to the doctor, especially to study behaviors or characteristics, in this case, illnesses clearly associated with the frequency of assistance with the medical consultations.
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(1) Also they say that to 22 % of the interrogated persons it imputes "the illness of the prostate". With the ENS we know that they say to have problems of prostate diagnosed by a doctor 7 % of the 16-year-old males or more, this is, a third. Diabetes would have 16 % of Lilly's study, opposite to 6 % in the ENS. And 37 % would have the high cholesterol, for 15 % in the ENS.