Adding SortedDictionary values

I'm trying to compute the Chi-Square values for a list of keywords.

It is strange that the chi-square values (sorted in alphabetical order of the keywords) are cumulative. I can't find where the problem lies after some troubleshooting. Hope that someone is able to help.

Please let me know if you need further clarifications on the code below. Thanks.

//ptxt is a SortedDictionary list <string keyword, string plaintext>foreach (string k in ptxt.Keys)
                {
                    totalLetChiSq = 0;
                    int pStrLen = 0;
                    int count = 0;
                    string kUpper = ptxt[k].ToUpperInvariant();

                    //Validate if each plaintext character matches the language letter frequency
                    foreach (char i in kUpper)
                    {
                        for (var j = 0; j < alphaArr.Length; j++)
                        {
                            if (i == alphaArr[j])
                            {
                                pStr += i;   //pStr contains only characters that are present in the language letter frequency
                                pStrLen++;          //Total no of characters for pStr
                            }
                        }
                    }

                    ptxtArrList = letterCount(pStr);    //Letter Frequency count for pStr

                    //Compute Chi-Square values for all the keywords
                    foreach (char i in ptxtArrList.Keys)
                    {
                        letChiSq.Clear();
                        numerator = ptxtArrList[i] - (langFreq[count] * pStrLen);
                        denominator = langFreq[count] * pStrLen;
                        cs = (Math.Pow(Convert.ToDouble(numerator), 2)) / Convert.ToDouble(denominator);
                        letChiSq.Add(i, cs);            // Store chi-sq value of each letter
                        totalLetChiSq += letChiSq[i];   // Compute chi-square value for a keyword
                        count++;    //increment counter for the next language letter frequency
                    }
                    try
                    {
                        KeysChiSq.Add(k, totalLetChiSq);
                    }
                    catch
                    {
                        MessageBox.Show("End of keyword file!");
                        break;
                    }

                    MessageBox.Show("Keyword " + k + " has a Chi-Square value of " + KeysChiSq[k].ToString());
                }
            }