Association and Causality, Faculty of Medicine, Epidemiology and Biostatistics, Lecture 1

Faculty of Medicine Epidemiology and Biostatistics (BMS222) ‫اﻟﻮﺑﺎﺋﯿﺎت واﻹﺣﺼﺎء اﻟﺤﯿﻮي‬ Lecture 1 Course Introduction Introduction to biostatistics Tariq N. Al-Shatanawi MPH PhD 14-7-2024 1  R...

Faculty of Medicine Epidemiology and Biostatistics (BMS222) ‫اﻟﻮﺑﺎﺋﯿﺎت واﻹﺣﺼﺎء اﻟﺤﯿﻮي‬ Lecture 1 Course Introduction Introduction to biostatistics Tariq N. Al-Shatanawi MPH PhD 14-7-2024 1  Record : - https://youtu.be/WGV-wYBzZzo?si=JSHKX6LoLt21dSXT  To find a statistical significant relationship True relationship between risk factor and disease >> between disease and exposure. A cause leads to disease. } Association = identifiable relationship (statistical connection) between an exposure and a disease } Causation states that there is a true relationship between risk factor and disease. A cause leads to disease. } An association alone does not make the relationship between risk factor and disease causal. - ‫ لحاله غير كافي يعرفني‬association‫ال‬ lead to disease‫ وال‬risk factor‫العالقة بني ال‬ like smoking Lung Cancer. } Ø If there is a difference in disease occurrence between groups, first question to be asked is whether this difference is statistically significant. If this difference is significant, then a statistical association is said to exist between the factor and the disease. 1. ‫ ولقينا في اختالفات‬variables‫ حسب نوع ال‬statistical analysis ‫اذا كان في عنا دراسة وعملنا‬ ‫ كان‬difference‫ فحتى نشوف هل هاد ال‬means and values that we measure‫ما بني ال‬ ‫ بحكي انو عندي‬significant ‫ وبس يطلع معي معي‬tests‫ الزم تعمل ال‬significant or not ‫ ابًدا‬cause ‫ فقط ما بحكي انو‬statistical association between factor and the disease This statistical association could be: 1. Artifactual (Spurious) association or 2. Indirect (non-causal) association. Or 3. Causal association  chance or bias‫ بكون خاطئ وطلع عن طريق ال‬significant ‫ يلي طلع عندي حتى لو كان‬association‫كل ال‬ Artifactual (Spurious) association is a false association that can result from chance or from bias in study methods. One should attempt to confirm this association by replication. If such an association doesn't hold up in this replication, then it may be considered as spurious. We do a lot of studies about the same issue >> Artifactual ‫ معناتو النتيجة يلي طلعت معي‬significant ‫ اذا ما كانوا‬statistically significant or not ‫هل كان هناك‬ Thus, whenever a statistically significant association is found, it must be examined carefully to be sure that it is not attributable to some artifact or bias. ‫ نأيدها بدراسات سابقة اثبتت نفس النتيجة‬significant ‫ ملا تطلع معي النتيجة‬research‫عشان هيك بال‬  ‫ طلع معي‬association‫ بس هاد ال‬statistically significant association ‫طلع معي‬ due to some common underlying condition Indirect(non-causal) association: It occurs when a factor and disease are associated only because both are related to some common underlying condition. Alteration in the indirectly associated factor will not produce alteration in the frequency of disease unless the change affects the common underlying condition as well.  Check next slide Example on indirect association: Altitude and cholera. Fetid air in low altitudes is the indirect factor. Water impurity in low altitude areas is the risk factor for cholera and not the fetid air. So low altitude is indirectly associated with cholera through the common underlying condition which is the water pollution.  upstream. The water in low altitude is more polluted than the upstream of the river. ‫املقصود انو املياه في مصب النهر ملوثة اكتر من املياه‬. ‫في اعلى النهر‬ fetid air هم كانوا رابطني ال‬ low altitude o C Significant association >> underlying factor that lead to cholera is polluted water not fetid air‫ولكن ال‬ fetid air‫ كان لها عالقة مباشرة مع ال‬water‫ولكن ال‬ polluted air‫ ولكنه ارتبط معها بسبب ال‬cholera‫ ما كان اله عالقة مباشرة مع ال‬fetid air‫ال‬  Risk factor a cause of this disease Causal association There are several criteria used to evaluate the likelihood that an association is causal: 1. Strength of the association as measured by the relative risk(Risk ratio) or the odds ratio. The larger this ratio, the greater the likelihood of this association to be causal.  < 2.0 very weak association 2.0 – 3.9 weak – moderate association > 4.0 moderate – strong association. ↑ incidence ↑ exposure.. ↑ ↑ Dose Response Direct relation Ship.. 2. Dose-Response relationship. With increasing levels of exposure to the factor, there is a corresponding rise in the disease. 3. Consistency of the association. An association discovered in one study persists in other studies conducted by using other methods and on different population. Association is observed repeatedly in different populations, different places and times, in different types of studies, conducted by different investigators All give the same relationship. First study and lack of consistency does not rule out causality ‫ رح نمشي عليها‬consistency‫املقصود لو كانت في كتير دراسات قبل اثبتت هاي ال‬ 4. Temporality. Exposure to the factor should precede the occurrence of disease and allows for the necessary period of induction or latency. disease‫ يسبق ال‬risk factor‫الزم يكون ال‬  S j99 j a Age-standardized death rates from J - bronchogenic carcinoma, by smoking history 11 19 sig - mortality rate n Adapted from Hammond EC, Horn D. JAMA 1958; 166:1294-1308.  !! ‫الدكتورة حكت االمثلة مش مطلوبة‬ & This specific risk factor lead to a specific disease 5. Specificity of the association. One manifestation follows from only one cause. Ex. Angiosarcoma of liver and exposure to vinyl chloride, adenocarcinoma of vagina in female offspring resulting from DES ingestion by mothers during pregnancy. ‫ يلي عم بدرسها مش مشكلة‬relationship‫بس لو ما كانت موجودة بال‬ Although specificity is a strong evidence for causality, yet lack of it is of less significance. 6.Coherence with existing information(Biological Plausibility). ‫هل النتائج يلي طلعت عندي ماشية مع العلوم‬ bladder cancer ‫ فهل بنقدر نحكي انو الدخان ال يسبب‬cancers ‫مثًال الدخان يؤدي لعدة‬ cancer ‫ بيعمل اكتر من نوع‬specificity ‫ال النو الدخان يسببه ولكن ما عنده‬  GoodLuck § Plausible – Higher incidence of disease in individuals who are more sexually active – The disease could be a sexually transmitted disease § Not Plausible – Polio vaccine reduces fertility Es – Conflicts with current knowledge of polio vaccine production, biological mechanism for fertility and contraception current Knowledge 1