Introduction to Statistics from a clinician1 - Variables

Ahmed Abdelaziz GhanemAssistant lecturer of General & Minimally Invasive surgeryMansoura University HospitalMansoura University, Egypt

Introduction

When we decide to do a study, we call it a trial . In any study there is the population, from which we choose a

sample to conduct the trial on. Data we want to collect is called the variables each of which has

a value .

Definitions Trial : the study we want to conduct. Population : is the entire pool from which a statistical sample is drawn, e.g.

( if study on HTN then population will be all people with HTN all over the world )

Sample : is a set of data collected and/or selected from a statistical population by a defined procedure, e.g. (if study on HTN them the sample will be the selected 100 cases under trial) .

Case : Every single object or subject.

Definitions

Variables : A variable is any characteristics, number, or quantity that can be measured or counted. A variable may also be called a data item. Age, sex, country of birth, eye color etc.

Value : each variable has a value which differs according to the variable.

Examples

A RCT for efficiency of new drug for treatment of hypertension. The study was done on 200 patients.

?? Trial ?? Population ??sample ??variables

Types of variables

Two Classifications : 1- Continuous Vs categorical ( Ordinal / Nominal ). 2- Dependent VS Independent.

Continuous variable

If a variable can take on any value between its minimum and its maximum value, it is called a continuous variable; otherwise, it is called a categorical variable.

For example : Age can be1 year = 12 m = 52 weakks = 365 d = no limit for expression between 2 points = continuous variable.

1 is the value, year is the discrimination .

Ordinal variable

At least 3 and limited . Containing hierarchy Example : stage of disease, Glasgow coma score , number of

pregnancies (no number between 2 and 3 no 2.5 pregnancy).

Nominal variable

mostly 2 values defined by their name: data can’t be ordered. Example : Group a and b / Male vs female / life vs dead /

Recovery vs No recovery . Or >2 but can’t be ordered e.g. race: Egyptian, Saudi, Libyan,

etc…

Advice

Biggest amount of information can be collected from continuous variable > ordinal > nominal .

In your trial try to collect continuous data.

Dependent Vs Independent

This classification is according to outcome : Primary outcome = dependent variable / Secondary outcome =

independent Example : A trial to study the probability of CHD Vs smoking * 2 variables : CHD and smoking ** We are testing the hypothesis of the probability of CHD so

CHD is the dependent variable. Furthermore we want to compare the probability of CHD among smokers Vs the probability among non-smokers so smoking status is the independent variable

Example: -in a RCT subject receives either : drug X or placebo (phantom–starch-sugar) to facilitate recovery , age and gender are confounding variables .

* 4 variables ? If the aim is to study the effect of the drug, What is the

dependent variable??

(Age) – (Gender) – (Recovery- No recovery ) – ( drug vs placebo) Recovery is the dependent variable if the aim is to study the

effect of the drug. If aim is the distribution of age: then age is dependent Variable is Dependent according to study question.

Excercise

Thank You