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Internal Validity: Definition and Examples - Statistics …


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External Validity Definition & Examples - Statistics How …

Threats to external validity compromise your confidence in stating that your study results are applicable to other situations. They are explanations of how you might be wrong in making generalizations. For example, your conclusion might be incorrect, the changes in the may not be due to changes in the , and variation in the dependent variable might be due to other causes. For example, may be competing with the independent variable to explain the study outcome.

Some specific examples of threats to external validity:

Internal Validity: Definition and Examples - Statistics How To

Validation is an empirical process accumulating evidence that a biomarker of exposure can be used either to assess past exposure or to predict expected outcomes. Such an empirical process encompasses several tiers, from the laboratory development to the field application. During a recent International Symposium on Biomarkers, a workshop was convened to assess the criteria and quality requirements for valid biomarker measurement. Quality assurance has been identified as a key issue or a pre-requisite for the validation of a biomarker of exposure. Quality assurance has been defined as the overall measures taken to ensure that laboratory results are reliable, including the adoption of scientifically sound criteria in the selection of the appropriate biomarker, and technically sound practices not only in the collection, transport, storage and analysis but also in the recording, reporting and interpretation of results. Quality control, either internal or preferably external, is an essential part of quality assurance, in that it aims at verifying that analytical results issued by the laboratory meet the requirements of the user (Aitio & Apostoli, 1995).

External Validity Definition & Examples - Statistics How To

Subsequent steps in the validation process include the critical assessment of different relationships: exposure-dose, dose-response (effect), effect-disease. Each one of such relationships may include the possible role of susceptibility biomarkers as potential confounders or modifiers. Whereas for short-lived organic compounds there are many examples of validation studies dealing with exposure-dose and only a few pertaining to dose-response relationships, the opposite is true for relatively long-lived metals. This means that data are often available for information that is easy to assess but unnecessary, whereas it is missing for crucial information, which may be difficult to obtain. Also, whereas in metal toxicology it is widely acknowledged that biomarkers of dose should not necessarily relate to recent exposure, but rather should predict adverse effects, biomonitoring of volatile organic compounds has mainly been used to demonstrate that internal dose reflects exposure. Since the latter can be easily measured, the role of biomarkers as a surrogate measure of something difficult or impossible to measure is questionable. Also, there are a number of situations where skin is a major route of absorption. Moreover, the use of personal protective devices at the workplace is growing and in such cases ambient exposure dose relationships are either difficult to assess or irrelevant.

Analytical validity refers to the properties of analytical methods, including their selectivity, dynamic range, inaccuracy and imprecision. Although simple colorimetric methods may still be used in a few instances, there is a tendency to rely on more sophisticated and costly techniques. Technology and powerful software applied to mass spectrometry are greatly improving hyphenated techniques, all relying on mass spectrometry (MS) detection. Hyphenated techniques take advantage of prior separation of analytes, but especially of: (i) the selectivity and sensitivity of MS; (ii) a wider dynamic range compared to suitable alternatives; (iii) little sample manipulation, which could give rise to artefacts. Whereas nanomolar concentrations can now be measured in biological media, new problems may arise in the control of pre-analytical factors and in the interpretation of the toxicological meaning of such low levels.

Classic threats to internal validity | awesomedogs

Validity has been defined as the degree to which the results of a measurement correspond to the true state of the phenomenon being measured. Another word for validity is accuracy. However, accuracy as well as other attributes often considered, such as sensitivity and specificity, may have different meanings depending on the scope. These terms accuracy, sensitivity and specificity may refer to inherent properties of the biomarker, i.e., to the accuracy with which a biomarker reflects the true exposure level, to its sensitivity in terms of changes associated with varying exposure levels, and to its specificity in terms of selectivity or source specificity. The same terms accuracy, sensitivity and specificity may refer to the validity of analytical methods, whereby accuracy is the ability to identify the true value of an accepted standard, sensitivity stands for the dynamic range of the method, and specificity for its ability to measure exclusively the chemical (parent or metabolite) of interest. Finally, accuracy, sensitivity and specificity may refer to the context of application, with regard to the ability of the biomarker to predict adverse effects, and to the number of false negative and false positive individuals in field applications.

A biomarker of effect has been defined as "a measurable biochemical, physiological or other alteration within an organism that, depending on magnitude, can be recognised as an established or potential health impairment or disease" (NRC, 1989). Research on biomarkers of effect is rapidly generating a large amount of data measuring intermediate end-points occurring probably after exposure and possibly before illness (Mutti, 1995). Such biomarkers are expected to reflect early modifications preceding progressive structural or functional damage at the molecular, cellular and tissue level. Therefore, they should identify early and reversible biochemical events that may also be predictive of later response (Mutti, 1991; Silbergeld, 1993). Unfortunately, the mechanism of action of many chemicals is unknown at present. Furthermore, changes occurring in target tissues or cells may not be mirrored by biochemical changes occurring in peripheral, accessible media. Finally, whereas early damage may be repaired and subsequent dysfunction compensated for, it may also trigger a "cascade of events" eventually leading to clinical disease (IPCS, 1991). However, there is no magic marker that can be used to distinguish reversible from progressive damage, but rather patterns that need to be identified on a one-by-one basis for each chemical and system or target organ (Mercier & Robinson, 1993).

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  • Internal Validity refers to a characteristic of a study’s design


  • Overview of Threats to the Validity of Research Findings


  • Internal and External Validity in Thought ..


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Internal Validity: Definition and Examples

The choice of the best approach depends very much on the mechanistic basis of adverse effects, which may be classified as: (i) acute or chronic (on the basis of triggering exposure patterns); (ii) local or systemic; (iii) early or delayed (from the triggering exposure); (iv) reversible or irreversible; (v) threshold (dose-related in terms of probability of occurrence and severity) or non-threshold (or stochastic, i.e., depending on the dose for the probability of occurrence but not for its severity). For acute and local effects, biomarkers of exposure may not be actually useful for preventive purposes. Chronic and irreversible adverse effects can result either from cumulated doses or from cumulated effects. Similar cumulated doses may result from repeated short-term, high-dose levels or from long-term, low-level exposures. Although it is often impossible to predict which one of these patterns is relevant to health risks, a relevant biomarker of exposure should serve as a bridge linking occupational or environmental pollution with a long-term health outcome or with some relevant intermediate end-point (Mutti, 1995). If the mechanism of toxicity is known, kinetic parameters are useful to identify the biomarkers suitable for assessing exposure levels that are thought to elicit the observed effects. It is generally assumed that the longer the half-life of a marker, the better is its correlation with most situations representing a matter for concern in public health, i.e., with effects resulting from chronic, long-term, low-level exposure to cumulative toxicants.

Reliability and Validity - Statistics Solutions

In the validation process, several questions must be addressed before biomarker(s) can be extensively used and accepted as scientific tools in quantitative risk assessment: (i) their intrinsic validity, in terms of relevance, stability, sensitivity, specificity, accuracy, and precision; (ii) their mechanistic basis, including application in relevant animal models; (iii) their advantages over other methods, if available, used to characterize exposure and effects; (iv) their interfering factors, which could act as confounders or modifiers; (v) their predictive and prognostic validity in field investigations with proper study design. Perhaps, no single available marker fulfils all of the above criteria. This justifies their prudent use in risk assessment.

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Three main strategies have been followed in developing biomarkers of effect: (i) epidemiological; (ii) clinical; (iii) experimental. Epidemiological studies have been useful to identify biomarkers that are associated, though not necessarily causally related, to later outcomes. Such a strategy is possible when the outcome is relatively frequent and multifactorial in nature, and the measured biomarker is inexpensive and readily available, e.g., serum cholesterol in cardiovascular disease. Most biomarkers of effect have been identified on the basis of pathophysiological reasoning, usually starting from clinical conditions, extrapolating backward changes preceding illness, e.g., early markers of nephrotoxicity. Since such markers are then used in epidemiological studies, a different methodological context of application may lead to misinterpretation of the health significance of observed changes, which greatly depends on the prevalence of the condition being examined (Mutti, 1993). The experimental approach is usually multi-tiered: studies and animal experiments are used to identify the mechanism of action of toxic chemicals; comparative studies are performed to verify that changes in candidate biomarkers occur both in target tissue and in peripheral, accessible media; epidemiological investigations are carried out to assess the sensitivity of such potential biomarkers to toxic chemicals. Such an approach appears to be the most relevant to immuno- and neurotoxicology, for in the immune and in the nervous system, potential target cells are either disseminated in the organism or confined to an inaccessible compartment. In both cases, function is regulated by the balance between triggering and inhibiting stimuli. Any alteration of such systems can hardly be interpreted in terms of toxicity in the absence of experimental models providing some mechanistic clues.

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