case studies: when you can't or won't run an experiment (and still want to study...

When you can't or

won't run an experiment (and still

want to study atypical language development)

David Saldaña

Case studies: why do them

• A case study is not just the study of a case

• What makes a case:

– Critical cases for a theory

– Unique or rare cases

– Representative cases

– Longitudinal case – time series

GOOD AND BAD CASE STUDIES Plan your study!

Construct validity

• Your measures should reflect what you say you are measuring

– Multiple sources of evidence

– Others review draft report

– Chain of evidence

Yin (2009)

Chain of evidence

Paper Database Protocol Questions

External validity

• Where can your findings be generalized?

– Use theory

– Use replication logic across cases

Yin (2009)

Reliability

• Would someone else find the same results with the same operations

– Detail a protocol

– Make sure you test interrater reliability

Yin (2009) © Nevit Dilmen via Wikimedia Commons

Internal validity

• Establish causal relationships, distinguished from spurious relationships

– Pattern matching

– Explanation building

– Addressing rival explanations

– Using logic models

Yin (2009)

ANALYZING THE DATA

Pattern Matching

• Put the info into different arrays

• Make a matrix of categories

• Graphically analyze the data

• Count!

• Do stats

• Use time

Pattern matching (more)

• Test predictions against your data – looking at the DV: – Predicted should happen

– NOT predicted should NOT happen

• Test independent variables against your data – looking at potential ID: – Predictor should be there

– NON predictor should NOT be there

• Find the simplest

Single-case studies:

• single-case studies typically employ a large number of measures,

• more often than not these measures are expressed on different metrics,

• there is an emphasis on examining the profile (i.e., the relative strengths and weaknesses)

• it is not uncommon for single-case studies to employ more than one control sample

Crawford et al., 2010

Effect sizes

Crawford et al., 2010

What should you report? At least

• the mean and standard deviation for controls on the task

• the raw score of the single case

• the effect size for the difference between the case and controls

Careful with inflation of type 1 error!

Crawford et al., 2010

Some software

http://homepages.abdn.ac.uk/j.crawford/pages/dept/psychom.htm

Thank you

http://institucional.us.es/dcptea

http://personal.us.es/davsalsag/inicio.html