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Data, Validitas & Reliabilitas data
Fakultas Kedokteran UnramBobby M. Syahrizal13 November 2007
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Health measurementHealth as the outcome/effect of exposure to a risk factor or a planned intervention (etiological, prognostic research)What makes a good outcome measure?Health as the absence or presence of disease to be assessed from certain indicators present with people who have complaints (diagnostic research)Quality/value of diagnostic test?Health as the probable presence or absence of disease, to be assessed by means of a certain test applied to (apparently) healthy individuals (early diagnosis, screening)What are useful screening test?
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Jenis Data/Skala PengukuranData kualitatif or categorical dataData which can be grouped based on certain categoryNominal:Binary or two category only exist, e.g. sex, yes/noUnordered or ranked category, e.g. blood group (A, B, A/B & O), type of cancer, raceOrdinal Ordered or ranked qualitative category, e.g. high, moderate & low
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.lanjutan jenis dataData kuantitatifCan be either measured (continuous) or counted (discrete)Discrete only absolute valuee.g. frekuensi kunjungan ke puskesmas, jumlah anak, frekuensi pemeriksaan ante natalContinuous can take any value within a given rangee.g. kadar kolesterol, age, height, weight, blood pressure
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.lanjutan jenis dataConsisted of Ratio with true zero point that reflects the absence of the measured characteristice.g. blood pressure, body weight, time, age, kelvintemperatureInterval without absolute zero value
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Exercise 1Specify each of the following variable whether nominal, ordinal, ratio or intervalHousehold income ($)Infant birthweight (grams)Systolic blood pressure (below normal, within normal range, above normal)Diastolic blood pressure (mm Hg)Parity (number of viable offspring produced by a woman)Drug dose (mg)Cancer staging (e.g. Dukes classification scheme)
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Exercise 2Twenty-five male vlunteers, ages 18 to 24 years, participated in a study to compare the effects of small doses of melatonin and a placebo to induce sleep at bedtime. Subjects who took melatonin fell asleep in 5 or 6 minutes; volunteers who received the placebo took 15 minutes or more to fall asleep.
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Validity
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ValidityValidity = accuracy the relative abstinence of systematic (non-random) errorsThe extent to which a device (e.g. a test) measures what it purports to measure, or a clinical observation accurately describes a phenomenonFor diagnostic or screening test, validity has 2 components: sensitivity & specificity
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Sensitivity & specicity
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What will happen if the same diagnostic tool applied in two different populations?What will happen if two different diagnostic tools applied in the same population?
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Prevalence = 50%Sensitivity = 50%Specificity = 50%Prevalence = 20%Sensitivity = 50%Specificity = 50%Prevalence = 20%Sensitivity = 90%Specificity = 50%Prevalence = 20%Sensitivity = 50%Specificity = 90%
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Exercise 3A physical examination was used to screen for breast cancer in 2,500 women with biopsy-proven adenocarcinoma o the breast and in 5,000 age- and race-matched control women. The results of the physical examination were positive (i.e., a mass was palpated) in 1,800 cases and in 800 control women, all of whom shoed no evidence of cancer at biopsy.Determine the sensitivity, specificity, positive and negative predictive value of the physical examination was?
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Exercise 4A physician visited his general internist for a regular annual medical examination, which included a stool examination for occult blood. One of the three stool specimens examined in the test was positive. The internist told his physician-patient that the result was of no significance because he regularly encountered many false-positive test results in his busy practice. The test was repeated, and all three stool specimens were now negative. Nevertheless, sensing his patients lingering concerns, the internist referred his physician-patient to a gastroenterologist. The gastroenterologist said, In my experience the positive stool finding is serous. Such a finding is almost always associated with gastrointestinal pathological disorders. The subsequent negative test mean nothing, because you could have a tumor that only bleeds intermittently.Whose judgment was correct in this occasion?
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ReliabilityPrecision, reproducibility, repeatabilityRelative absence of random errorRegardless of the sensitivity and specificity of a test, if the test results cannot be reproduced, the value and usefulness of the test are minimal
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Sources of variabilityBiological variationWithin individualsBetween individualsErrors in measurement toolSame instrumentDifferent instrumentsRater/observer errorSame rater/observerDifferent rater/observer
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Biological variation
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Exercise 5 at homeHow can we measured validity when there is no gold standard instrument as a comparison?How can we assess inter-observer variation?
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Iridology: does it look well?Within the domain of alternative medicine iridology is applied as a procedure to diagnose (the predisposition to) certain disorders. The whole procedure (looking into a persons eye) rest on the assumption that many organs of the human body are connected with the iris of the eye and are projected into it through nervous paths. Disorders of these organs are said to be expressed by typical changes in particular structures of the iris.Several iridologists are practicing also in the Netherlands. The are making good business. One of the reasons for their popularity is the attractive layout of the diagnostic procedure they rely on. Internal organs are approached by means of a non-invasive technique and the patient doesnt suffer any physical pain, at worst only in his wallet. Iridology: Ideal or ideology?Iridologists: can they look their clients straight into their eyes?
