What are personal errors?

What are different types of errors measurements made by instruments?

A person looking at a person wearing a shirt with a pattern comprising vertical and horizontal lines is able to see the vertical lines more distinctly than the horizontal ones. What is this defect due to? How is such a defect of vision corrected?

what do you mean by least count error?

What is meant by random errors ?

What is an error? How are they classified and how do the errors combine?

What are various types of systematic errors ?