Olá Mario;
Eu tenho feito algo parecido para a soma em intervalos de 10 em 10,
Adaptei aqui a um campo novo na consulta e agrupei para os seus 15 em 15, teste para ver se funciona;
O novo campo ficará assim;
Minutos: IIf(Val(Right([Hora];2))>=0 And Val(Right([Hora];2))<=14;"[00-14]";IIf(Val(Right([Hora];2))>=15 And Val(Right([Hora];2))<=29;"[15-29]";IIf(Val(Right([Hora];2))>=30 And Val(Right([Hora];2))<=44;"[30-44]";IIf(Val(Right([Hora];2))>=45 And Val(Right([Hora];2))<=59;"[45-59]";""))))
No SQL da consulta fica:
SELECT tblExemplo.Data, Left([Hora],2) AS Hor, IIf(Val(Right([Hora],2))>=0 And Val(Right([Hora],2))<=14,"[00-14]",IIf(Val(Right([Hora],2))>=15 And Val(Right([Hora],2))<=29,"[15-29]",IIf(Val(Right([Hora],2))>=30 And Val(Right([Hora],2))<=44,"[30-44]",IIf(Val(Right([Hora],2))>=45 And Val(Right([Hora],2))<=59,"[45-59]","")))) AS Minutos, Count(tblExemplo.TC_STAT) AS Total
FROM tblExemplo
GROUP BY tblExemplo.Data, Left([Hora],2), IIf(Val(Right([Hora],2))>=0 And Val(Right([Hora],2))<=14,"[00-14]",IIf(Val(Right([Hora],2))>=15 And Val(Right([Hora],2))<=29,"[15-29]",IIf(Val(Right([Hora],2))>=30 And Val(Right([Hora],2))<=44,"[30-44]",IIf(Val(Right([Hora],2))>=45 And Val(Right([Hora],2))<=59,"[45-59]",""))));