When there are insufficient sample occurrences of a context string, build-imm employ additional criteria to determine Y {\displaystyle Y} value. For a given context string S x , i {\displaystyle S_{x,i}} of length i, build-imm compare the observed frequencies of the following base f ( S x , i , a ) {\displaystyle f(S_{x,i},a)} , f ( S x , i , c ) {\displaystyle f(S_{x,i},c)} , f ( S x , i , g ) {\displaystyle f(S_{x,i},g)} , f ( S x , i , t ) {\displaystyle f(S_{x,i},t)} with the previously calculated interpolated Markov model probabilities using the next shorter context, I M M i − 1 ( S x , i − 1 , a ) {\displaystyle IMM_{i-1}(S_{x,{i-1}},a)} , I M M i − 1 ( S x , i − 1 , c ) {\displaystyle IMM_{i-1}(S_{x,{i-1}},c)} , I M M i − 1 ( S x , i − 1 , g ) {\displaystyle IMM_{i-1}(S_{x,{i-1}},g)} , I M M i − 1 ( S x , i − 1 , t ) {\displaystyle IMM_{i-1}(S_{x,{i-1}},t)} . Using a X 2 {\displaystyle X^{2}} test, build-imm determine how likely it is that the four observed frequencies are consistent with the IMM values from the next shorter context."[1]