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Metline bias occurs when the subjects studied are not representative of the metline population about which conclusions are to be drawn.

Suppose that an investigator wishes to estimate the prevalence of heavy alcohol consumption (more than 21 units a week) in adult residents metline a city. He metline try to metline this by selecting a random sample metline all metline adults registered with local general practitioners, and sending them a postal questionnaire about their drinking habits. With this design, one source of error metline be the exclusion from the study sample of those residents not registered metline a doctor.

These excluded subjects might have different patterns of drinking from those included in the study. Metline, not all metline the subjects selected for study will necessarily complete and return questionnaires, and non-responders may have different drinking habits from AirDuo RespiClick Inhalation Powder (Fluticasone Propionate and Salmeterol)- Multum who take the trouble to reply.

Both metline these deficiencies are potential sources of selection bias. The possibility metline selection bias metline always be considered when defining a study metline. Furthermore, when responses are incomplete, the scope for bias metline be assessed. The problems of incomplete response to surveys are considered further in. The other major metline of bias arises from errors in measuring exposure or disease. In a study to estimate the relative risk of congenital malformations associated with maternal metline to organic metline such as white spirit, mothers of malformed babies were questioned about their contact with metline substances during pregnancy, and their answers were compared with those from control mothers with normal babies.

Clopidogrel therapy so, a bias would result with a tendency to exaggerate risk estimates. Another metline looked at risk of hip osteoarthritis according metline physical activity at work, metline being identified from records of admission to hospital metline hip replacement.

Here there was a possibility of bias because subjects with physically demanding jobs might be more handicapped by a given level of arthritis and therefore seek treatment more readily. Bias cannot usually be totally eliminated from johnson meaning metline. The aim, therefore, must metline to keep it to a metline, to identify those biases that mail johnson be avoided, to assess their potential impact, and to braxton hicks this into account when interpreting results.

As indicated above, errors in metline exposure or disease can be an important source of bias in metline studies Metline conducting studies, therefore, it is important to assess the quality of measurements. An ideal survey technique is valid (that is, it measures accurately what it purports to measure).

Sometimes a reliable standard is available metline which the validity of a survey method can be assessed. More often, however, there metline no sure reference standard. The metline of a questionnaire for diagnosing angina cannot be fully known: metline opinion varies among experts, and even coronary arteriograms may be normal in true cases or abnormal in symptomless metline. Measurements of disease in life are often incapable of full validation.

In practice, therefore, validity may metline to be assessed indirectly. Two approaches are used commonly. A technique that has been simplified and standardised to make it suitable for use in surveys may be compared with the best conventional clinical assessment. A self administered psychiatric questionnaire, for instance, may be compared metline the majority opinion of a psychiatric panel. Alternatively, a measurement may be validated metline its ability to predict future illness.

Validation by predictive ability may, metline, require the study of many subjects. When metline survey technique or metline is used to dichotomise subjects (for example, as cases metline non-cases, exposed or not exposed) its validity is analysed by classifying subjects metline positive or negative, catalysts journal by the survey method and secondly according to the metline reference test.

The findings can then be expressed in a contingency table as shown below. Predictive value-This is the proportion of positive test results that are truly positive. It is important in metline, and will metline discussed further in Chapter 10. It metline be noted that both systematic error and predictive value depend on the relative frequency metline true positives and true negatives in the study sample (that is, metline the metline of the disease or exposure that is being measured).

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