5.2 Characteristics of Data: Types of Studies

Suppose you had observed that the heights of Grade 9 students were quite varied, but by the time these students reach Grade 12, most are taller. State a thesis question that gives your study a focus and identifies the population.

What is the pattern of height growth of students, from the time they enter high school until they graduate? Who is the target of the study? –Any students? –Only 1 st and last year? –Treat females and males alike? Can we make an inference about growth patterns beyond the school? First Try:

We need to define the population more clearly. What is the pattern of growth of female and male students at Jarvis from the time they enter high school until they graduate? Note: what if they enter Jarvis at age 17?

How will we answer the question? –Full population or sample? –What is the time-frame? We need to know what kind of study we are going to do.

Types of Studies Census –Every person/thing in the population is surveyed/looked at –(Government census every 6 years) Sample Survey –Only a small selection of the population

Pros and Cons Census Pros –Can make perfect statements about the population Census Cons –Difficult to conduct –Costly (time, resources) to conduct and process data Sample Survey Pros –Relatively inexpensive (time, resources) to conduct and process data –If sample well chosen, can make good inferences to population Sample Survey Cons –If sample not well chosen, inferences might be incorrect

Types of Studies Cross-sectional –Focuses on a wide range of individuals at different stages –Carried out once –E.g.: Measure the heights of the 2007 grade 9 class and compare them to the heights of the 2007 grade 12 class.

Types of Studies Longitudinal –Focuses on the same small group of individuals over a period of time –Usually a choice for medical/psychological study –E.g.: Measure the heights of the 2007 grade 9 class. Measure the heights of those same students again four years later.

Pros and Cons Cross-Sectional Pros –Quick –Easy –Relatively inexpensive –Good for overall generalizations Cross-Sectional Cons –Not comparing the exact same people –Generalization might be off –There might be other factors influencing results Longitudinal Pros –Comparing the exact same people Longitudinal Cons –Expensive –Difficult to undertake –Study subjects might drop out

Time Series Data: –Data that have been accumulating over a long period of time –Ex: How a person’s opinion changes over time Earthquake data from 1905 onward

A good thesis –States the population –States the main variables –Has variables that are measurable and give enough data –States the kind of study

Let’s try this again: What is the pattern of growth of female and male students at Jarvis from the time they enter high school until they graduate, as shown by comparing a random sample of 2007 grade 9 students to a random sample of 2007 grade 12 students?

Pg. 91 #14 (a) (b) (c) (d) (e) Population: Cars in N.A. Rate of depreciation of foreign cars (quantitative) Rate of depreciation of domestic cars (quantitative) sample continuous Cross-sectional. Why? Cheaper and easier. Impractical to follow a bunch of cars for 5-10 years!