Count of customers whose rating is lowered more than 3 this year - dax

Question

I am here with a DAX/PBI question.
I have a table that contains Customer number, rating date and rating. (which is shown below). Each customer has several rating notes in different dates, as shown below:
CustomerNumber RatingDate Rating
887 01.10.2020 2
887 31.12.2019 5
887 06.01.2020 3
887 31.12.2018 3
940 10.07.2020 14
940 31.12.2019 10
656 01.05.2020 8
656 31.12.2019 8
I want to create a measure and place it on a card that shows the count of customers whose rating dropped more than 3 levels in this year (compare between last rating in 2019 and last rating in 2020).
As in this example table, result should be 1 (cus. number 940)
However the measure that i wrote doesnt seem to be working: (blank result)
COUNTROWS(
FILTER('Rating Change';
IF(COUNTROWS(FILTER('Rating Change';'Rating Change'[Ratingdate].[Year]=2020))=0;BLANK();
IF(COUNTROWS(FILTER('Rating Change';'Rating Change'[Ratingdate].[Year]=2019))=0;BLANK();
CALCULATE(MAX('Rating Change'[Rating]);FILTER('Rating Change';'Rating Change'[Ratingdate].[Year]=2019)) -
CALCULATE(MAX('Rating Change'[Rating]);FILTER('Rating Change';'Rating Change'[Ratingdate].[Year]=2020))>3
)
)

Answer 1

Probably you have an error in your sample data, because 940 has increased his Level;
The solution could look like this:
measure = countrows(FILTER (
ADDCOLUMNS (
VALUES ( 'Rating Change'[CustomerNumber] ),
"maxRating",
CALCULATE (
MAX ( 'Rating CHange'[Rating] ),
FILTER (
ALL ( 'Rating Change'[RatingDate] ),
VAR _x =
CALCULATE (
MAX ( 'Rating Change'[RatingDate] ),
YEAR ( 'Rating Change'[RatingDate] ) = YEAR ( TODAY () )
)
RETURN
'Rating Change'[RatingDate] = _x
)
),
"maxRatingLY",
CALCULATE (
MAX ( 'Rating CHange'[Rating] ),
FILTER (
ALL ( 'Rating Change'[RatingDate] ),
VAR _x =
CALCULATE (
MAX ( 'Rating Change'[RatingDate] ),
YEAR ( 'Rating Change'[RatingDate] )
= YEAR ( TODAY () ) - 1
)
RETURN
'Rating Change'[RatingDate] = _x
)
)
),
[maxRatingLY] - [maxRating] > 3
)
)