I need to calculate the change between records and use the result elsewhere, but the field does not list the summary function in the shortcut menu and if I create a formula I get the "this field cannot be summarised" error"

My dataset is a list of rates at various dates.
Fields include:
@vol_mid = ({volrates.vol_bid}+{volrates.vol_offer})/2
@Previous record = Previous({@vol_mid}) (this could be the source of the error since I cannot summarise this field).
@log_dailychange = log({@vol_mid}/{@Previous record})
@sd_change = stddev(@log_dailychange) ....error = "this field cannot be summarised"

Q1: how can I use summary functions on @previous record
Q2:how can I calculate the stddev for the daily change

I read on other threads that the whileprintrecords plus a variable is used to sum the variable but I have no idea how this would work for a stddev.

Is there an easier/any way to do this. I am not a programmer by nature so I have limited knowledge in areas where you do not see the functions in the formula editor.

Thank you for your assistance
 

Thank you for the reply, I hoped there was an easier way though. Could you please help me with getting the previous value using the variable method?

I dont seem to get it working
for @vol_mid i used
whileprintingrecords;
shared numbervar thisvar;
thisvar:=(({volrates.vol_bid}+{volrates.vol_offer})/2)

for @previousrecord I tried
whileprintingrecords;
shared numbervar lastVal;
shared numbervar thisVar;
if NOT ISNULL(thisVar) then
  lastVal := thisVar;
the error is @field is required here (referrign to 'thisVar')

I tried the following but then @previous = @vol_mid
whileprintingrecords;
shared numbervar lastVal;
shared numbervar thisVar;
 
if recordnumber = 1 then lastval:=0 else
    lastval:=thisvar

I think I need to code the following logic but I am not finding a solution
if recordnumber = 1 then thisvar=@volmid and lastval=0
else lastval = thisvar (value of the first record) , thisvar should tehn change to the value of the current@volmid and the next lastval = thisvar.
 every way I code it thisvar assumes the @volmid of the current record and not of the previous record.

what am I missing?

No, it returns FALSE. The problem I found with having thisvar anywhere else but where it is linked to record 1, is that it takes the value of the current record's @vol_mid, somehow you need to specify that on hte first record thisvar must be @vol_mid but lastval = 0
on the following record thisvar starts off as the @vol_mid of record 1 and then lastval = thisvar but agter writing this value thisvar now need to change  to @vol_mid of the current record.