Lexile varies depending upon sigma value which varies from 1.25 to 0 and as the number of days between test keep on increasing sigma is increased.
Before the test begins, LP needs to estimate the student’s ability so it knows how hard to make the first item. It also needs to set a start value for Sigma.(based on mathematical calculations) According to the theory, as when more question is asked the uncertainty will decrease. Thus the score will become more accurate.
Now, in many cases students are taking test for their 2nd, 3rd or even nth time, and system assumes that student reading ability increases moderately throughout the school year. For example, it assumes that students will gain about 12L per month in grades 3-5 and about 4L per month in grades 8+. In light of this, system adds between 4L and 32L to the student’s last score for each month that has elapsed since their last print test or computer test.
So why there is drop of 100 L when gap is of 90 days, should it not rather increase?
The answer to this is below -
If the Lexile will increase then question level will increase. If sigma value also increase then uncertainty will also increase, that means if the student make the first answer wrong, then its Lexile score will decrease by 20 to 300 points and student’s Lexile score will increase if he gives correct answer.
So to the customer we can say “when student appears for next test with a gap of 60+ day, system assumes that student has done enough practice to grow their comprehension level, system assumes that student reading ability increases moderately throughout the school year and Lexile rating gets a little stringent, however question level is still based on student’s ability and this can be avoided by giving the next test in about 30 to 50 days.