I. Sampling
A. Samples vs. Populations
*Sample = group of people participating in the study
*Population = group of people to whom I want to generalize the tresults
-target population - how well does it respresent the populaton you're trying to address
-accessible population (200 kids)
*Two types of sampling:
A.
Probability Sampling (text calls it simpel random sampling) = (pulling a random group of people in such a way that each inidividual has an equal chance of being selected for participation in the study. (
could also be called a straight random sampling)Probability Sampling Methods:
1. Stratified Random Sampling= select subsets of the population to participate in the study in the same proportion as they appear in the population
e.g., 400 teachers in Salt Lake area schools, 275 are female and 125 are male
I decide to sample 40% of Salt Lake area teachers. My sample contains:
40% * 400 teachers = 160 total teachers in sample
40% * 275 female teachers = 110 females in sample
40% * 125 male teachers = 50 males in sample
2. Clustered random sample= select existing groups of participants instead of creating subgroups
e.g., Instead of randomly selecting individuals in correct proportions, I randomly select groups of individuals
So now I randomly select some schools in Salt Lake area district, and all teachers in those selected schools participate in my study
But, I must ensure that those groups selected are representative of my population as a whole.
3. Two-Stage Random Sampling (will probably never see this but should know what it is) = combines methods 1 (stratified) and 2 (clustered); in stage 1, existing groups are randomly selected; in stage 2, individuals from those groups are randomly selected
e.g., Instead of randomly selecting individuals in correct proportions, I randomly select groups of individuals, then randomly select individuals from those groups
Stage 1: I randomly select some schools in Salt Lake area district. (first pick my cluster, then stage 2 is picking my people from those clusters)
Stage 2: From each selected school, I randomly select a subset of teachers to participate in the study
B.
Non-Probability (or non random) Sampling (also not always used) = individuals are selected from the population in such a way that not everyone has an equal chance of being selected for participation in the study. (such as choosing every 3rd M&M in a group)
1. Systematic Sampling (the best way to study)= every nth individual in a population is selected for participation in the study
e.g., I take an alphabetical list of all teachers in Salt Lake area schools, and select every 3rd individual from that list for participation in my study. Here, 3 is my sampling interval
**** (Means every what is being sampled) sampling interval = population size / desired sample size
e.g., sampling interval = 400 teachers / 160 teachers (or 40%) =2.5
sampling ratio = proportion of individuals in population selected for sample
e.g., sampling ratio = 160/400 = .4 or 40%
2. Convenience Sampling = select from a group of individuals who are conveniently available to be participants in your study
e.g., I go into schools at lunchtime and give surveys to those teachers who can be found in the teachers’lounge
Potential Problem:
Sample is likely to be biased –are those teachers in the lounge at lunchtime likely to be different from those who aren’t?
This type of sampling should be avoided if possible.
3. Purposive Sampling (be alert to this kind of sampling, biased)= researchers use past knowledge or own judgment to select a sample that he/she thinks is representative of the population
e.g., I decide to just give my survey to teachers who are also currently enrolled in the EDPS 63030, because I *think* they are representative of the population of Salt Lake area teachers
Potential problem: Researchers may be biased about what they believe is representative of a population, or they may be just plain wrong.
**Beware of this type of sampling as the potential for bias is very strong.
***First question you should ask aobut sampling: Does it represent the target population?II. Sampling in Qualitative Research • Purposive Sampling - i will select those individuals that I think represent the interest
• Case Analysis (can be any of the following)–Typical (teacher burnout: typical example would be a teacher who is just tired)
–Extreme (teacher burnout: extreme example is the teacher who burned out so bad who went and got a job at Blockbuster or became a scuba instructor)
–Critical (portrays the critical lessons of burnout, usch as not carrying anymore, come up with reasons to cancel class, use the same stuff every time, going through the motions until retirement)
• Maximum Variation (someone who is really low in burnout, makes all other teachers sick b/c won't retire b/c they just love it so much so they teach 45 years)• Snowball Sampling ( start off with a small group of participants and ask those people to ask other people to get more people, like a snowball rolling down the hill that gets bigger and bigger the longer it rolls).Sampling and Validity1. What size sample is appropriate?Descriptive => 100 subjects
Correlational=> 50 subjects
Experimental => 30 subjects per group (often see less numbers than this)*
Causal-Comparative => 30 subjects per group*
But if groups are tightly controlled, less (e.g., 15 per group) may be OK.
2. How generalizable is the sample? external validity= the results should be generalizable beyond the conditions of the individual study
1. Population generalizability= extent to which the sample represents the population of interest
2. Ecological generalizability= degree to which the results can be extended to other settings or conditions
III. What is Measurement? • Measurement (just the collection of data)
• Evaluation (making decisions on the basis of the collected data)
• Where does assessment fit in?
A. What kind of scale is the measurement based on?•
Nominal - refers to the same thing as categorical variables, things you can name, things that are categories, such as gender (qualitative variable)
•
Ordinal - rank ordering of information (quantitative); no information, just an order, such as 1, 2, 3; we don't know the distance between 1 and 2, who voted for who, just what rank they made
•
Interval - you do get information in regards to the distance between a rank of 1-2.) On an interval scale, you can measure distance; if a room is zero degrees in Fahrenheit, does that mean there's a complete absence of heat? No, that's an interval scale,
no absolute zero!•
Ratio - there is an absolute zero when measuring.IV. Types of Educational Measures• Cognitive vs. Non-Cognitive Cognitive = interested in the learning process
Non-Cognitive=
• Commercial vs. Non-Commercial
- Commercial: probably the norm, been tested, been standarized, but may not be tailored to the specific interest of the research study
-Non-Commercial: materials can be developed specifically for the research study
• Direct vs. Indirect
Direct just get the information directly from the participant
Indirect: getting information from somebody else, such as doing a documents analysis to understand principles, where the documents may or may not come from the participant
V. Sample Cognitive Measures •
Standardized Tests –Achievement Tests
–Aptitude Tests
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Behavioral Measures –Naming Time
–Response Time (how quickly does one respond to a question)
–Reading Time (how quickly can one read a passage or a word)
•
Wpm •
Eyetracking Measures (enables the researcher to determine exactly where the person is looking on a keyboard/computer down to the exact keystroke) determines how you look at images vs text
***AIO = area of interest
We'll do section VI later in the semester
VI. Non-Cognitive Measures • Surveys & Questionnaires
• Observations
• Interviews
How is an individual’s score interpreted? 1.
Norm-referenced instruments= an individual’s score is based on comparison with peers (e.g., percentile rank, age/grade equivalents, grading on curve, etc.)
2.
Criterion-referenced instruments= an individual’s score is based on some predetermined standard (e.g., raw score)
Interpreting Data Different Ways to Present Scores: 1.
Raw Score= number of items answered correctly, number of times behavior is tallied, etc.
2.
Derived Scores= scores changed into a more meaningful unit
a. age/grade equivalent scores= for a given score, tell what age/grade score usually falls in
b. percentile rank= ranking of score compared to all other individuals who took the test
c. standard scores (
same as a z score)= indicates how far scores are from a reference point; usually best to use in research
e.g. the mean is 560 overall for a test; a classroom mean is 140 and the student gets a 132. how do you compare the two? using a
Z score or a standard score puts both groups on the same scale, so you can compare apples and oranges.
Important Characteristics of Measures •
Objectivity - all measures must be objective in order to get a good measure (using a rubric)
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Usability - needs to be usable, need to know how to use it
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Validity - refers to does the meausre actually measure what it is suppose to measure.(such as does a reading comprehension test really measure reading comprehension or is the student just comprehending what they are looking for by looking for the question specificallt when they read)
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Reliability - do I get consistent measurement over time? Without reliability you don't have anything!!!!!
