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SECONDEDITIONOFTHEALARM INTERNATIONALPROGRAMSYLLABUS

Data Presentation in the ALARM International Program

In providing maternity care, we strive to produce the most desirable results for our patients with minimum risks

and costs. When information needed to make rational decisions is nonexistent, incomplete or unavailable, ourdecision-making is severely hampered. When information exists and is not used properly or consistently,inappropriate care may be provided. The goal of the AA!" International #rogram is to promote care based onthe best available evidence while also encouraging participants to develop their skills in obtaining, evaluating andincorporating evidence into daily clinical practice.

Definitions

$. Sensitivityis the likelihood that the diagnostic test will indicate the presence of disease when the disease isactually present. %True positive rate&. 'a(%a)c&*

+. Specificityis the likelihood that the diagnostic test will indicate the absence of disease when the disease isactually absent. %True negative rate&. 'd(%b)d&*

. Positive predictive valueis the likelihood that a positive test result actually means that the disease is present.'a(%a)b&*. Negative predictive value is the likelihood that a negative test result actually means that the disease is absent.

'd(%c)d&*. Bayes' Theorem/ The predictive value of a test will depend on the prevalence of the disease. With high

prevalence, the positive predictive value will increase and vice versa %i.e. in a high prevalence setting, apositive test result is more likely to indicate a true positive, than in a low prevalence setting&

0or example, in the clinical setting of doing a culture for 1roup 2 3trep %123&/

3ensitivity of the test % )ve culture& is the chance that if the woman had 123 it would be picked up by the test

3pecificity is the chance that the test will indicate no 123 %-ve culture& when in fact the woman does not have

it

#ositive predictive value is the chance that a )ve culture represents 123 coloni4ation

5egative predictive value is the chance that a -ve culture actually rules out 123

Clinical Trials

A prospective !andomi4ed 6ontrolled Trial %!6T&, which controls for known and unknown variables betweenthose receiving and those not receiving a given intervention, is the most powerful way to discover if theintervention has a significant impact. Trials are more likely to show a difference between the intervention andcontrol group, if it truly exists, when the numbers studied are large and when the difference in outcome is great.

In obstetrics, most serious outcomes are rare. This means it can be very difficult or impossible to carry out

ALARM INTERNATIONALData Presentation

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randomi4ed controlled trials that are powerful enough %have enough sub7ects& to demonstrate a difference betweena treatment and control group, even if it exists. Trials that have been done are often too small alone to providestatistically significant results. "eta analysis is one tool that may allow useful information to be obtained fromthese studies.

A meta analysis is a systematic evaluation of a collection of several studies which are similar in design, studypopulations and outcomes examined. 2y combining data appropriately, the answer to an important 8uestion maybe found in the cumulative information in the medical literature. A secondary benefit of meta analysis is thesystematic review of the medical literature that occurs by the analysts who retrieve and synthesi4e the informationand make it more readily available for our use.

In meta analysis, the data are presented as follows/

Odds Ratio

The odds ratio %9.!.& compares the likelihood of the outcome being studied occurring in the group receiving theintervention %the :experimental; or :exposed; group& with the group not receiving the intervention %the :control;or :unexposed; group&.

9.!. < 9bserved outcome in experimental group < a x d 9bserved outcome in control group b x c

The odds ratio serves as a surrogate for the relativerisk which is more difficult to manipulate usingthe statistics performed in meta analysis.

1raphically, the odds ratio is presented as a point on a hori4ontal, logarithmic scale. A vertical line drawn at $indicates no difference in the outcome between the two groups. !atios less than one will be represented to the leftof the vertical line and those greater than one will be represented on the right side of the vertical line. The data

presentation is usually constructed so that the results that are less than one are an improvement in outcome.

Confidence IntervalsThe confidence interval is a measure of statistical significance, generally calculated as the least and greatest odds

ratio within which the 9.!. results of the experiment would fall => of the time. It is displayed graphically as ahori4ontal line through the 9.!. point where the left end represents the lowest and the right end represents thehighest odds ratio. The => confidence interval is e8uivalent to the probability statistic p?@.@.


OUTCOME

EXPOSUR

E


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"easurements of confidence do not eliminate the possibility that the results of an experiment are due to chance,they 7ust indicate how likely it is that such a result is due to chance. The 7udgment of the clinician is re8uired tointerpret whether or not a result is clinically significant regardless of the statistical expression of probability used.

9utcome A

9utcome 2

9utcome 6

9utcome

9utcome B

0or a single trial, the outcomes of interest are shown with their individual odds ratio and confidence interval.

Typical Odds Ratio

In a meta analysis, the results of individual studies will usually be put on a graph above the calculated statistic, thetypical odds ratio. Thus, the typical odds ratio represents the calculated odds ratio when all the trial results are

pooled for a single given outcome of interest. The associated confidence interval will be narrower than theindividual trials because the number of participants represented is greater.


CBxposedC CDnexposedC

Odds Ratio (95% Confidence Interval

!eceived treat ment 6ontrol

reatment sig nificantlyetter than control

reatment sign ificantlyorse th an control

confidence

interval

odds ratio

Eesults consistent

ith chance


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TABLE - Effect of Intervention on Outcome of Interest

Trial A %n < ++@&

Trial 2 %n < $&

Trial 6 %n < @&

Trial %n < +@&

Trial B %n < +&

Typical odds ratio

The confidence intervals displayed for each study will be broader than the confidence interval of the typical oddsratio and vary according to study si4e. In the example above/ Trial 6 has the narrowest confidence interval givenits larger si4e. The typical or FpooledG odds ratio is the narrowest of all as it mathematically incorporates all of thetrialsG sub7ects. At times, a trial may fail to demonstrate a difference that truly exists due to a lack of power %i.e.not enough sub7ects given the effect si4e& H Trial in the above example. Again, confidence intervals that crossthe vertical line at $ means that p @.@ for the outcome in that trial. Although this means that this result is notstatistically significant, if all the studies lie to one side of the vertical axis, it indicates a trend in the samedirection. The trials are then said to be homogeneous. This suggests that a difference may truly exist and may

become apparent once the typical odds ratio is calculated or more studies are added to the analysis.

The results displayed in a meta analysis fall into one of three categories/

The 9.!. lies to the left of the vertical axis %one& and the confidence interval does not cross one. This indicatesthat the outcome for the treated group is less likely to occur than in the control group and the result is statisticallysignificant.

The 9.!. is at or near one and the confidence interval line crosses one. This indicates that there is no statisticallysignificant difference in outcomes between the groups.

The 9.!. lies to the right of the vertical axis and the confidence interval line is also completely to the right of one.This indicates that the outcome is more likely to occur in the treated group than in the control group.

Bven though several studies may not all achieve statistical significance individually, perhaps because some haveinsufficient numbers or the effect si4e is small, the meta analysis display may show most of these studiesdemonstrate the same trend. Dnder this circumstance, when the typical odds ratio and its confidence intervaldemonstrates a significance difference, one may be more confident that this difference truly exists.

0or example, several studies were conducted regarding the use of antepartum glucocorticoids on fetal lung maturityand the occurrence of neonatal respiratory distress syndrome. 3hown are the lead author, year of study publicationand number of sub7ects in each trial.

iggins, $=J+ %n


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2lock, $=JJ %n

data presentation2002

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