Experimental design

EXPERIMENTAL DESIGNS COMPLETELY RANDOMIZED DESIGN, RANDOMIZED BLOCK DESIGN, LATIN SQUARE DESIGN , OTHER DESIGNS.

D. Kavitha

MSc(N); MSc(Psy);BSAM; DHHM Ph.D Scholar

Experimental Research

Research design

It is master plan specifying the methods and procedures for following for collecting and analyzing the needed information in a research study

Experimental research design

…the researcher selects participants and divides them into two or more groups having similar characteristics and, then, applies the treatment(s) to the groups and measures the effects upon the groups

Uniqueness of experimental research design

• Experimental Research is unique in two important respects:1) Only type of research that attempts to

influence a particular variable2) Best type of research for testing hypotheses

about cause-and-effect relationships• Experimental Research looks at the

following variables:• Independent variable (treatment)• Dependent variable (outcome)

Major Characteristics of Experimental Research

• The researcher manipulates the independent variable.

• They decide the nature and the extent of the treatment.

• After the treatment has been administered, researchers observe or measure the groups receiving the treatments to see if they differ.

• Experimental research enables researchers to go beyond description and prediction, and attempt to determine what caused effects.

Essential Characteristics of Experimental Research

Comparison of Groups:• The experimental group receives a treatment of some

sort while the control group receives no treatment. • Enables the researcher to determine whether the

treatment has had an effect or whether one treatment is more effective than another.

Manipulation of the Independent Variable:• The researcher deliberately and directly determines what

forms the independent variable will take and which group will get which form.

Essential Characteristics of Experimental Research

Randomization• Random assignment is similar but not identical to random

selection.

• Random assignment means that every individual who is participating in the experiment has an equal chance of being assigned to any of the experimental or control groups.

• Random selection means that every member of a population has an equal chance of being selected to be a member of the sample.

• Three things occur with random assignments of subjects:1) It takes place before the experiment begins2) Process of assigning the groups takes place3) Groups should be equivalent

Simple Random Sample

Every subset of a specified size n from the population has an equal chance of being selected

Stratified Random Sample

The population is divided into two or more groups called strata, according to some criterion, such as geographic location, grade level, age, or income, and subsamples are randomly selected from each strata.

Cluster Sample

The population is divided into subgroups (clusters) like families. A simple random sample is taken of the subgroups and then all members of the cluster selected are surveyed.

Systematic Sample

Every kth member ( for example: every 10th person) is selected from a list of all population members.

Types of Designs

The basic structure of a research study . . . particularly relevant to experimental research

Types of experimental designs (Campbell & Stanley, 1963)

Pre-experimental Quasi-experimental True experimental

Pre-experimental design

Quasi –experimental design

True experimental design

•One shot case design•One group pretest-posttest design•FEATURES•Manipulation of independent variables•Limited control over the extraneous variables•No randomization and control group

•Non randomized block design•Time series design•FEATURES•Manipulation of independent variable•Absence of either randomization/ control group

•Post –test only control design•Pre –test– posttest control group design•Factorial design•Randomized block design•Cross over design•FEATURES•Manipulation of independent variable •Presence of control group•Randomization

Variable

a concept (e.g., intelligence, height, aptitude) that can assume any one of a range of values

Independent variable - an activity of characteristic believed to make a difference with respect to some behavior

Ex - experimental variable, active variable, cause, treatment

Dependent variable - the change or difference occurring a result of the independent variable

Ex- Assigned variable, effect, outcome, posttest

Steps in conducting experimental research

Decide if an experiment addresses the research problem

Form hypotheses to test cause-effect relationships

Select an experimental treatment and introduce it

Identify study participants choose a type of experimental design

Conduct the experiment Organize and analyze the data Develop an experimental research report

The concept of validity…the experiment tests the variable(s) that it purports to test

Threats to validity… Internal: factors other than the independent

variable that affect the dependent variable( campbell 1963)

External: factors that affect the generalizability of the study to groups and settings beyond those of the experiment

Threats of internal validity

History Maturation of subjects Testing Instrumentation change Mortality Selection bias – maturation interaction

History

Some event beside the experimental treatment occurs during the course of the study , and this event even influence dependent variable.

Maturation of subjects

Experimental research is carried on long period of time over a group of subjects there may be changes in the subjects in different ways.

Increase in height, weight. Ex. Nutritional protocol on height &

weight of malnourished children

Testing

Effect of taking a pretest of subjects’ performance of post test.

The effect of taking a pretest may sensitize an individual and improve the score of the post test.

Individuals generally score higher during second test regardless of treatment.

Instrument change

Changes in instruments, calibration of instruments, observers or scorers may cause changes in the measurements

Mortality

Loss or dropout of the subject during course of the study

The longer period of study the more chance for dropout.

Ex. longitudinal study

Selection bias

Subjects are not selected randomly for participation in groups , there is a possibility of comparison may not equivalent.

External validity

Hawthorne effect Subjects may behave in particular

manner because they are aware that they are being observed

Experimental effect

Threat to study results when researcher’s characteristic , mannerism, behavior may influence subject matter.

Reactive effect of pretest Effect of pretest occurs when subjects

have been sensitized to the treatment because of taking pretest.

Ex – pretest may sensitize to learn about HIV/ AIDS irrespective of health education is provided

Novelty effect:

Treatment is new , the subjects and researchers act different ways

People : Generalization is not applicable depending upon the race.

Place: Generalization not possible for people living in rural and urban area

Time : older results can not be generalized over periods of time.

Most common way to eliminate threats

Experimental control Experimental control attempts to predict events that will occur in the experimental setting by neutralizing the effects of other factors.

Physical Control Gives all subjects equal exposure to the independent variable. Controls non-experimental variables that effect the dependent variable.

Selective Control Indirectly manipulate by selecting in or out variables that cannot be controlled

Statistical Control Variables not conducive to physical or selective manipulation may be controlled by statistical techniques.

Criteria for evaluating experimental Research

Does the experiment have a powerful intervention?

Does it employ few treatment groups (e.g. only two)?

Will participant profit from the intervention?

Is there a systematic way the researcher derived the number of participants per group?

Criteria for evaluating experimental Research

Were there an adequate number of participants used in the study?

Were valid, reliable, and sensitive measures or observations used?

Did the study control for extraneous factors?

Did the researcher control for threats to internal validity?

Types of pre experimental design

The One-Shot Case Study A single measure is recorded after the

treatment in administered. Study lacks any comparison or control of

extraneous influences. To remedy this design, a comparison could

be made with another group. Diagrammed as:

The One-Group Pretest-Posttest Design

Subjects are measured before and after treatment is administered.

Uncontrolled-for threats to internal validity exist.

To remedy this design, a comparison group could be added.

Diagrammed as:

The Static-Group Comparison Design

Use of 2 existing, or intact groups. Experimental group is measured after being

exposed to treatment. Control group is measured without having

been exposed to the treatment. Diagrammed as:

The Static-Group Pretest-Posttest Design

Pretest is given to both groups. “Gain” or “change” = pretest score -

posttest score. Better control of subject

characteristics threat. A pretest raises the possibility of a

testing threat.

Pre experimental design

Advantages DisadvantagesVery simpleConvenient to conduct in natural settings

Suitable for beginners

Weak design to establish casual relationship between independent and dependent variable

Very little control over the research

Higher threat to internal validity

Characteristic of quasi experimental research design

Manipulation of independent variable Lack of one / two essential character of

true experimental design Quasi independent variable used instead

of true independent variable.

Types of quasi experimental design

Nonequivalent /Non randomized control group design

O X O O Orandom assignment of intact groups that are

pretested ( O ), exposed to a treatment ( X ) and then posttested ( O )

Time-series design O O O O X O O O O a single group is pretested ( O ) repeatedly until

pretest scores are stable, exposed to a treatment ( X ) and, then, is repeatedly posttested ( O )

Possible Outcome Patterns in a Time-

Series Design

Characteristics of true experimental design

Manipulation – control of independent variable by the researcher through treatment/ intervention

Control – the use of control group and extraneous variables on the dependent variable

Randomization – every subject gets equal chance being assigned to experimental and control group.

Advantages Disadvantages

Most powerful design to establish causal relationship between independent and dependent variable

Cannot be replicated in studies conducted in human begins due ethical problems

Purity of the observation Many of the human variables neither have valid measurable criteria nor instruments to measure.

Create conditions in a short period of time that may take years to occur naturally

Studies conducted in hospital / community difficult to control the extraneous variable

Conducted in laboratory, experimental unit, specialized research setting

Very difficult get co operation for treatment/ intervention

True Experimental

• The essential ingredient of a true experiment is random assignment of subjects to treatment groups

• Random assignments is a powerful tool for controlling threats to internal validity– The Randomized Posttest-only Control Group

Design• Both groups receiving different treatments

– The Randomized Pretest-Posttest Control Group Design

• Pretest is included in this design– The Randomized Solomon Four-Group Design

• Four groups used, with two pre-tested and two not pre-tested

The Randomized Posttest-Only Control Group Design

Experimental group tested after treatment exposure.

Control group tested at the same time without exposure to experimental treatment.

Includes random assignment to groups.

Threats to internal validity – mortality, attitudinal, implementation, data collector bias, location and history.

Example of a Randomized Posttest-Only Control Group

Design

The Randomized Pretest-Posttest Control Group Design

Experimental group tested before and after treatment exposure

Control group tested at same two times without exposure to experimental treatment

Includes random assignment to groups.

Pretest raises the possibility of a pretest treatment interaction threat

Example of a Randomized Pretest-Posttest Control Group

Design

The Randomized Solomon Four-Group Design

Combines pretest-posttest with control group design and the posttest-only with control group design.

Provides means of controlling the interactive test effect and other sources of extraneous variation.

Does include random assignment.

Weakness: requires a large sample.

Example of a Randomized Solomon

Four-Group Design

A Randomized Posttest-Only Control Group Design

Solomon four-group design R O X1 O

R O X2 O

R X1 O

R X2 O

four groups are formed by random assignment ( R ) of participants, two groups are pretested ( O ) and two are not, one pretested and one un pretested group receive the experimental treatments ( X1, X2 ), each group is are administered a posttest on the dependent variable, and posttest scores are compared to determine effectiveness of treatments

Factorial design

involve two or more independent variables with at least one independent variable being manipulated by the researcher

two-by-two factorial design (four cells)

2 X 2 two types of factors (e.g., method of

instruction) each of which has two levels (e.g., traditional vs. innovative)

Using a Factorial Design to Study Effects of Method and Class Size

on Achievement

Illustration of Interaction and No Interaction in a 2 by 2 Factorial Design

Example of a 4 by 2 Factorial Design

Randomized block design Principle of local control along with other

two principle of experimental design subjects are first divided into groups each group the subjects are relatively

homogeneous The number of the equal in each group Extraneous variable is fixed

Type of antihypertensive drugs

Blocks

Patients with primary hypertension

DM patients with hypertension

Renal patients with hypertension

A A,I B,II A,III

B B,I B,II B,III

C C,I C,II C,III

Cross over design / repeat measure design

Subjects exposed more than one treatment

Subjects randomly assigned to different orders of treatment

Equal distribution of character among the group

Latin square design

very frequently used in agricultural research.

An experiment has to be made through which the effects of five different varieties of fertilizers on the yield of a certain crop.

out put occur depend on soil not only on the fertilizer

L.S. design is used when there are two major extraneous factors such as the varying soil fertility and varying seeds

SeedDifferences

FERTILITY LEVEL

X1 A B C D E

X2 B C D E A

X3 C D E A B

X4 D E A B C

X5 E A B C D

Other designs

Descriptive design Univariant descriptive design – the

frequency of occurrence of the phenomenon Ex – the experience of patients suffering

from rheumatoid arthritis Prevalence of vitamin D deficiency among

pregnant women Used to identify, describe the perception,

awareness, behavior, attitude, knowledge and practice of people.

Exploratory design

Used to identify , explore and describe the existing phenomenon and its related factors

Ex . contributing factors of sleep disturbance among patients admitted in ICU

Comparative design

Comparing and contrasting two or more sample of subjects on one or more variable

Attributes-Knowledge, perception, attitudes

Physical and psychological symptoms Ex KAP on Vitamin D among antenatal

mothers

Prospective Cohort Study

Some have the factor (c)

Population

(lapse of time)

Begin enquiry here& work forwards

Sample people without

the disease

Disease (a)

Disease (b)

No Disease

No Disease

Statistic = Relative Risk [RR] = (a/c) divided by (b/d) This shows the ratio of incidence in exposed

compared to non-exposed.RR > 1 implies a hazard;

RR < 1 implies a protective factor

95% CI are usually presented:e.g., RR = 1.9 (95% CI 1.5, 2.3)

Note: as you beginwith people who do nothave the disease, youcan calculate incidencebut not prevalence.(Prevalence would beunderestimated as youomitted existing cases)

Some do not (d)

Outcomes

Retrospective Case-Control Study

Population

SelectCases

(have the disease)

Sample ofControls

(who do not have the disease)

Exposed (c)

Exposed (a)

Not Exposed (d)

Not Exposed (b)

Begin enquiry here& look backwards

Statistic = Odds Ratio [OR] = (a/b) divided by (c/d)This shows how many times more likely were the cases

to have been exposed than the controls.OR interpreted in same way as RR

Reviewhistory

Reviewhistory

Note: as you beginwith people who alreadyhave the disease, youcannot calculateincidence or prevalence

Developmental research design

Cross sectional design Researcher collect data at particular

point of time Ex –assessing the awareness on swine

flu among people of an areaLongitudinal design Collect the extended period of time follow up studies

Other type of trails

Pilot studies and feasibility studies– run before a large trail take place

Screening trails – cervical cancer screening trail

Prevention trails – breast cancer prevention trail.

Trails looking at causes and patterns of disease

Case control studies Sequential trails

Conclusion

There are several research designs and the researcher must decide in advance of collection and analysis of data as to which design would prove to be more appropriate for his research project.

Applying What you Have Learned: An Experimental Study

Review the article and look for the following: The research problem and use of quantitative

research Use of the literature The purpose statement and research

hypothesis Types and procedures of data collection Types and procedures of data analysis and

interpretation The overall report structure

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