- coeff = pca(X) returns the principal component coefficients, also known as loadings, for the n - by - p data matrix X. Rows of X correspond to observations and columns correspond to variables.
What is the difference between observation and variable in case of pca() function?
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K M Ibrahim Khalilullah
am 22 Nov. 2017
Kommentiert: Star Strider
am 23 Nov. 2017
I have a matrix with m rows and n columns that is built from n number of individuals for person identification. So, n is the number of person and m is the number of feature's value of the person or pixel's intensity values of the person's image.
Now I want to calculate pca using Matlab's pca() function. But, It makes me confused about observation and variables. What will I call n and m? Which one represents observation and which one represents variable?
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Star Strider
am 22 Nov. 2017
From the documentation:
So each row is an observation and each column is a variable.
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Star Strider
am 23 Nov. 2017
My pleasure.
Principal components analysis is quite good for that. I actually once used it to design a filter when I did a linear discriminant analysis on short-time Fourier transforms of EEGs to classify what task the person was doing. The filter then isolated the relevant frequencies. making the classification much more efficient.
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Image Analyst
am 23 Nov. 2017
The person is treated as a variable and the feature value is the observation. The feature values for each person are listed in a column of the table. For example, maybe you have two people in a family and want to see if there is a relationship between the weight of the two people over time. You might be taking measurements every day (or month or whatever). So if you had 4 time points and 2 people, the array would be
w11, w21
w12, w22
w13, w23
w14, w24
where w1* are the weights of person #1, and w2* are the weights of person #2. The coefficients would be
coefficients = pca(weightsMatrix);
Where weightsMatrix is that 2-D array I gave above.
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Image Analyst
am 23 Nov. 2017
Please describe the measurements you are making. If the ID of persons is your observation then you may need logistic regression instead of PCA.
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