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