Low power objective lens

The odds ratio for each table is 1.0, and the Mantel summary adjusted odds ratio is 1.0. The crude odds ratio and the Mantel-Haenszel summary odds ratio are quite different (2.26 and 1.0), concluding that smoking was a confounding factor and there appears to be no association (odds ratio = 1.0) between alcohol and MI in this example. Note that the odds ratio in the two strata is the same (1.0). There is no interaction or effect modification between smoking and alcohol. In other words, the effect of alcohol on MI is the same for smokers and nonsmokers. When the effect varies in the different strata (the odds ratios are different), interaction or effect modification is present.

Parfocal: the objective lenses are mounted on the microscope so that they can be interchanged without having to appreciably vary the focus.

Note: Summary results only appear in stratified data sets (summary results show adjusted results based on the stratification criteria).

Objective lensmagnification

The Mantel-Haenszel weighted odds ratio, risk ratio, summary chi square, and p-value calculations combine results from different strata to remove confounding caused by the variables used for stratification. If tables are created for male and female, confounding by sex is removed. The degree of confounding can be judged by comparing the crude and weighted odds ratios. If the ratios are identical, there was no confounding by sex.

The following example investigates the relationship between alcohol and myocardial infarction (Relationship Between Alcohol Consumption and Myocardial Infarction (MI): Confounding Due to Smoking Hypothetical Data, Schlesselman JJ. Case-control studies. New York: Oxford University Press, 1982).

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High power objective microscope function

High power objectivemagnification

If the odds ratio or risk ratios for strata in a series of stratified tables for the tests are not similar, then interaction between the stratifying factor and the risk factor are present.

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Ocular lensmagnification

Total magnification: In a compound microscope the total magnification is the product of the objective and ocular lenses (see figure below).  The magnification of the ocular lenses on your scope is 10X.

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Low power objectivemagnification

Stratifying a dataset separates the population into distinct categories based on levels of a parameter (i.e., sex). If confounding is present, associations between disease and exposure can be missed or falsely detected. A confounding factor is one that is associated with the disease and the exposure, but may not be of interest or observed. Age is a frequent confounder, although any factor other than the main exposure being considered can be treated as a confounder.

Smoking is known to be associated with MI and alcohol consumption. Stratifying the data by smoking status creates two tables, one for smokers and one for nonsmokers.

Stratification means making a separate table of disease by exposure for each possible confounder combination. In the simplest case, this could mean separate male and female tables if sex is the potential confounder. If age, sex, and city are confounders, separate tables will be made for each possible combination of age group, sex, and city.

Immersion Oil:  Clear, finely detailed images are achieved by contrasting the specimen with their medium.  Changing the refractive index of the specimens from their medium attains this contrast.  The refractive index is a measure of the relative velocity at which light passes through a material.  When light rays pass through the two materials (specimen and medium) that have different refractive indices, the rays change direction from a straight path by bending (refracting) at the boundary between the specimen and the medium.  Thus, this increases the image’s contrast between the specimen and the medium.

Resolving power or resolution: the ability to distinguish objects that are close together.  The better the resolving power of the microscope, the closer together two objects can be and still be seen as separate.

The Adjusted Mantel-Haenszel and Adjusted Mantel-Haenszel confidence limits provide additional measures. If the weighted odds ratio or risk ratio (not for case-control studies) has confidence limits that do not include 1.0, there is a statistical association with 95% confidence between the disease and the exposure without confounding by the stratifying factor.

One way to change the refractive index is by staining the specimen.  Another is to use immersion oil.  While we want light to refract differently between the specimen and the medium, we do not want to lose any light rays, as this would decrease the resolution of the image.  By placing immersion oil between the glass slide and the oil immersion lens (100X), the light rays at the highest magnification can be retained.  Immersion oil has the same refractive index as glass so the oil becomes part of the optics of the microscope.  Without the oil the light rays are refracted as they enter the air between the slide and the lens and the objective lens would have to be increased in diameter in order to capture them.  Using oil has the same effect as increasing the objective diameter therefore improving the resolving power of the lens.