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Lack of independent replicates: A common pitfall in experimental design.

Lack of independent replicates: A common pitfall in experimental design. Replication. the number of individual samples included in an average greater replication provides greater statistical strength reduces variance; strengthens the generality of the mean. Replicates must be Independent.

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Lack of independent replicates: A common pitfall in experimental design.

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  1. Lack of independent replicates: A common pitfall in experimental design.

  2. Replication • the number of individual samples included in an average • greater replication provides greater statistical strength • reduces variance; strengthens the generality of the mean

  3. Replicates must be Independent • all statistical analyses assume replicate values are independent observations • a single value does not depend on one, more than it does on another • failures of the assumption of independence • lack of truly random sampling or assignment to treatment groups • pseudoreplication (Hurlbert, S.H. 1984. Ecological Monographs 54(2): 187-211)

  4. Inhibition of Biofilm Formation Chemical 1 Chemical 2 Chemical 3 Control

  5. Replicates are not independent because they are all in the same dish Differences between dishes will be misinterpreted as differences between treatments

  6. Inhibition of Biofilm Formation Chemical 1 Chemical 2 Chemical 3 Control

  7. PseudoreplicationStuart Hurlbert. 1984. Ecological Monographs 54(2): 187-211 • Treating multiple measurements from/on the same sample the same as single measurements made from/on multiple samples • The multiple measurements are not independent because they are all from the same sample; hence they are not replicates

  8. At which temperature will Serratia marcescens grow best? 20C 25C 35C 45C Measure absorbance of five samples from each flask.

  9. Pseudoreplication because. . . • Multiple measurements from the same flask are treated as if they were single measurements from multiple flasks • Each flask represents a single replicate, because multiple samples taken from each flask are not independent of one another

  10. Five flasks incubated at each temperature. 20C 25C 35C 45C Measure absorbance of one sample from each of the 20 flasks.

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