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TREND ANALYSIS

TREND ANALYSIS

Sometime with repeated measures data is it useful to look at the trend in the data - something that can be achieved with polynomial contrasts.

Load the data HERE into a table and plot it using the following commands:

my.data = read.table('sampleRMData.txt')
names(my.data) = c('Subject','Week','RT')
my.data$Subject = factor(my.data$Subject)
my.data$Week = factor(my.data$Week)
plot(my.data$RT~my.data$Week)
Picture
So, imagine this is performance data and you want to show a trend in the data - for example a quadratic trend to relate this to the Power Law of Practice. You can do that easily with the emmeans package and polynomial contrasts.

Try the following code:

install.packages('emmeans')
library('emmeans')
myRMANOVA = aov(RT ~ Week + Error(Subject / Week), data = my.data)
print(summary(myRMANOVA))
weekContrast = emmeans(myRMANOVA,"Week")
poly = contrast(weekContrast,'poly')
print(poly)


You should see the following output:
Picture
So, you have a significant repeated measures ANOVA, given the data, not surprising. However, you also have significant linear and quadratic trends! You could report these to argue practice effects as opposed to reporting the results of paired sample t-tests.
  • NEUROSCIENCE
    • NEUROSCIENCE 100 >
      • NEURO 100 INTRODUCTION
      • NEURO 101 ADVANCED
      • NEURO 102 AGING
      • NEURO 103 MEMORY
      • NEURO 104 DECISION MAKING
      • NEURO 105 LEARNING
      • Research Statistics
    • NRSC 500B / MEDS 470
  • Kinesiology
    • EPHE 245
    • EPHE 357
  • STATISTICS
    • BIOMEDICAL STATISTICS
    • MULTIVARIATE STATISTICS >
      • MULTIPLE REGRESSION
    • RESOURCES
    • R TIPS
  • MATLAB
    • THE BASICS >
      • Hello World
      • BASIC MATHEMATICS
      • VARIABLES
      • Matrices
      • Writing Scripts
      • PATHS AND DIRECTORIES
      • USER INPUT
      • FOR LOOPS
      • WHILE LOOPS
      • IF STATEMENTS
      • RANDOM NUMBERS
    • STATISTICS >
      • LOADING DATA
      • DESCRIPTIVE STATISTICS
      • MAKING FUNCTIONS
      • BAR GRAPHS
      • LINE GRAPHS
      • TTESTS
    • EXPERIMENTS: THE BASICS >
      • DRAWING A CIRCLE
      • DRAWING MULTIPLE OBJECTS
      • DRAWING TEXT
      • DRAWING AN IMAGE
      • PLAYING A TONE
      • KEYBOARD INPUT
      • BUILDING A TRIAL
      • BUILDING TRIALS
      • NESTED LOOPS
      • RIGHT OR WRONG
      • SAVING DATA
    • EXPERIMENTS: ADVANCED >
      • STROOP
      • N BACK
      • Oddball
      • Animation
      • VIDEO
    • EEG and ERP Analysis >
      • ERP Analysis
      • FFT Analysis
      • Wavelet Analysis
  • Directed Studies
    • Advanced Topics in Motor Control A
    • Advanced Topics in Motor Control B
    • An Introduction to EEG
    • Advanced EEG and ERP Methods
    • Neural Correlates of Human Reward Processing
    • Independent Research Project
  • RESOURCES
    • EXCEL
    • HOW TO READ A RESEARCH PAPER
    • HOW TO WRITE A RESEARCH PAPER
  • Workshops
    • Iowa State EEG Workshop 2018
  • Python
    • The Basics >
      • Setting Up Python
      • Hello, world!
      • Basic Math & Using Import
      • Variables
      • Matrices
      • Scripts
      • User Input
      • For Loops