SQL & PL/SQL

Hi,
Below one is my query,when count isnull then i need to return 0(zero) can some one help

SELECT COUNT(*) TOT_STUD_ITEM_SS , 
                                                            Count(Case When RESPONSE_IS_CORRECT = 1 Then 1 End)  As TOT_STUD_CORR_ANS_ITEM_SS
                                                             FROM PSYCHOMETRIC_ITEM_STAT_OPN A, VW_SCALE_SCORE_CONFIG B
                                                             WHERE    SS_ITEM_STAT_OPN.ITEM_ID=TV_SS_ITEM_STAT_OPN(LN_SSI_OPN).ITEM_ID
                                                             AND     (SS_ITEM_STAT_OPN.BEGIN_VALUE=TV_SS_ITEM_STAT_OPN(LN_SSI_OPN).BEGIN_VALUE
                                                             AND    SS_ITEM_STAT_OPN.END_VALUE=TV_SS_ITEM_STAT_OPN(LN_SSI_OPN).END_VALUE)
                                                              AND     SS_ITEM_STAT_OPN.ITEM_ID = A.ITEM_ID 
                                                              AND SS_ITEM_STAT_OPN.END_VALUE = B.END_VALUE
                                                              AND ROUND(A.SCALE_SCORE) > BEGIN_VALUE
                                                              AND ROUND(A.SCALE_SCORE) <= END_VALUE
                                                              AND CONDITION_COLUMN= 'SCALE_SCORE'
                                                              GROUP BY ITEM_ID,END_VALUE

and i tried like below but it's still returning null only

SELECT nvl(COUNT(*) ,0) TOT_STUD_ITEM_SS , 
                                                             nvl(Count(Case When RESPONSE_IS_CORRECT = 1 Then 1 End),0) As TOT_STUD_CORR_ANS_ITEM_SS