Help with REGR_SLOPE function

I am trying to use the REGR_SLOPE function, but I am getting null values back from the function. I am probably making a simple mistake, but could do with some help understanding where I am going wrong:

SQL> select * from test_slope;

   SL_AGE    SL_MEAN
---------- ----------
         0      56.67
        20     292.33
        40        548
        60        713
        80     943.67

SQL> select sl_age, sl_mean, regr_slope(sl_age, sl_mean) 
over (partition by sl_age) "SLOPE" from test_slope;

   SL_AGE    SL_MEAN      SLOPE
---------- ---------- ----------
         0      56.67 
        20     292.33 
        40        548 
        60        713 
        80     943.67 

 

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I am expecting to get the following SLOPE values:

 SL_AGE    SL_MEAN      SLOPE
---------- ---------- ----------
         0      56.67       
        20     292.33      11.78
        40        548      12.28
        60        713      11.12
        80     943.67      10.97

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