Subtracting Cumulative Data

Hello - this is how you can do it in Power Query.  This solution uses ImkeF's custom function for referencing the previous row.

let
    fn = (MyTable as table, MyColumnName as text) =>
    let
        Source = MyTable,
        ShiftedList = {null} &  List.RemoveLastN(Table.Column(Source, MyColumnName),1),
        Custom1 = Table.ToColumns(Source) & {ShiftedList},
        Custom2 = Table.FromColumns(Custom1, Table.ColumnNames(Source) & {"Previous Row"})
    in
        Custom2 ,
    documentation = [
    Documentation.Name =  " Table.PreviousRow ",
    Documentation.Description = " Superfast way to reference previous row ",
    Documentation.LongDescription = " Superfast way to reference previous row ",
    Documentation.Category = " Table ",
    Documentation.Source = " www.TheBIccountant.com .  http://tiny.cc/hhus5y . ",
    Documentation.Version = " 1.0 ",
    Documentation.Author = " Imke Feldmann: www.TheBIccountant.com. http://tiny.cc/hhus5y . ",
    Documentation.Examples = {[Description =  "  ",
    Code = "  ",
    Result = "  "]}],
    Result = Value.ReplaceType(func, Value.ReplaceMetadata(Value.Type(func), documentation)),
    
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("hdFBDoMgEIXhu7A2gXkDU3sWw/2vUSACUp/tYoKLL8gPx+HEbQ7w0LKKjx4BKJ+pTd6+QepAqpBAiHWCsG6iHnH+peKmCEgd7GWMAOtA4hDgJa82dzBL6jGVCFta0xT3kLbHeRvgKY+bjJbLfSlvsRGrP1oiEeujnCfV/zH6EFOfQ4hYY94u5w8=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"Contract Number" = _t, #"Financial Year" = _t, Date = _t, #"New Patients" = _t, #"Outcome Required" = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Contract Number", Int64.Type}, {"Financial Year", type text}, {"Date", type date}, {"New Patients", Int64.Type}, {"Outcome Required", Int64.Type}}),
    #"Sorted Rows" = Table.Sort(#"Changed Type",{{"Contract Number", Order.Ascending}, {"Financial Year", Order.Ascending}, {"Date", Order.Ascending}}),
    #"Grouped Rows" = Table.Group(
        #"Sorted Rows", 
        {"Contract Number", "Financial Year"}, 
        {
            {"Data", each fn(_, "New Patients") }
        }
    ),
    #"Expanded Data" = Table.ExpandTableColumn(#"Grouped Rows", "Data", {"Date", "New Patients", "Outcome Required", "Previous Row"}, {"Date", "New Patients", "Outcome Required", "Previous Row"}),
    #"Added Custom" = Table.AddColumn(#"Expanded Data", "New", each [New Patients] - ( [Previous Row] ?? 0 ))
in
    #"Added Custom"

Hi, 

Another solution with Table.NestedJoin

let
Source = YourSource,
Index = Table.AddIndexColumn(Source, "Index", 0, 1, Int64.Type),
Next_Month = Table.TransformColumns(Index,{{"Date", each Date.AddMonths(_,1), type date}}),
NestedJoin = Table.NestedJoin(
      Index, {"Contract Number", "Financial Year", "Date"},
      Next_Month, {"Contract Number", "Financial Year", "Date"}, "Prev_Month", JoinKind.LeftOuter),
Expand = Table.ExpandTableColumn(NestedJoin, "Prev_Month", {"New Patients"}, {"New Patients.1"}),
Subtracting = Table.AddColumn(Expand, "Outcome Required", each List.Sum({[New Patients], -[New Patients.1]}), type number),
Sort = Table.Sort(Subtracting,{{"Index", Order.Ascending}}),
SelectColumns = Table.SelectColumns(Sort,{"Contract Number", "Financial Year", "Date", "New Patients", "Outcome Required"})
in
SelectColumns

Stéphane