请教连老师和各位，我做面板门限时，设定 bs1(300) bs2(300) bs3(300)，可以得到结果，但设定改为500时，出现：matsize too small    You have attempted to create a matrix with too many rows or columns or attempted to fit a model with too many variables.  You need to
    increase matsize; it is currently 410.  Use set matsize; see help matsize.

    If you are using factor variables and included an interaction that has lots of missing cells, either increase matsize or set emptycells  drop to reduce the required matrix size; see help set emptycells.

    If you are using factor variables, you might have accidentally treated a continuous variable as a categorical, resulting in lots of  categories.  Use the c. operator on such variables.
我已set matsize 8000
请问，这是什么原因；factor variables是什么意思？
谢谢！