In[9]:= p = Predict[{{1.3, "P"} -> 1, {1.8, "Q"} -> 2.5, {1.9, "Q"} -> 3, {0.2, "P"} -> 1, {-3.2, "P"} -> -4.2, {0.3, "Q"} -> 2}];
In[10]:= PredictorInformation[p, "Function"]
Out[10]= 0.452129 - 0.530351 Boole[#2 === "P"] + 0.530351 Boole[#2 === "Q"] + 1.12488 #1 &