diff --git a/src/main/java/org/example/Administration.java b/src/main/java/org/example/Administration.java
index 95f0e2c..8e6053b 100644
--- a/src/main/java/org/example/Administration.java
+++ b/src/main/java/org/example/Administration.java
@@ -188,25 +188,23 @@ public class Administration {
             {
                 //the entered id is used
                 System.out.println("Please enter an ID which is not being used!");
-                correctData = false;
+
             }
             else if(findProfessorById(UserId) == null || findAdminById(UserId) == null || findStudentById(UserId) == null)
             {
                 //the entered id is not used
 
-                //creates a substring to check if the id-"syntax" is correct
-                String substring = UserId.substring(1);
 
                 if(!rightPrefix(UserId, userType))
                 {
                     //the prefix is wrong
-                    correctData = false;
+
                 }
-                else if(!isNumber(substring))
+                else if(!isNumber(UserId))
                 {
                     //the substring does not contain only digits
                     System.out.println("Every character besides the first one need to be digits");
-                    correctData = false;
+
 
                 }
                 else{
@@ -238,15 +236,18 @@ public class Administration {
 
     /**
      *
-     * @param substring the userID without the prefixes S (for Student ), A ( Admin), P (Professor)
+     * @param userId the userID
      * @return returns true if the substring is only containing digits, returns false either the substring is empty or includes not only digits
      */
-    public boolean isNumber(String substring)
+    public boolean isNumber(String userId)
     {
-        if(substring == null || substring.isEmpty())
-        {
-            return false;
-        }
+        //creates a substring to check if the id-"syntax" is correct
+        String substring = userId.substring(1);
+
+        //if(substring == null || substring.isEmpty())
+        //{
+        //    return false;
+        //}
         for(char c : substring.toCharArray())
         {
             if(!Character.isDigit(c))
@@ -267,53 +268,50 @@ public class Administration {
     {
         switch (i){
         case 1: //Student
-            if(userId.startsWith("S"))
+            if(!userId.startsWith("S"))
             {
-               return true;
+                System.out.println("The ID needs to start with a capital S!");
+                return false;
             }
             else if(userId.startsWith("s"))
             {
                 System.out.println("The ID needs to start with a capital S!");
                 return  false;
             }
-            else if(!userId.startsWith("S"))
+            else
             {
-                System.out.println("The ID needs to start with a capital S!");
-                return false;
+                return true;
             }
-            break;
-        case 2: //Admin
-            if(userId.startsWith("A"))
+            case 2: //Admin
+            if(!userId.startsWith("A"))
             {
-                return true;
+                System.out.println("The ID needs to start with a capital A!");
+                return false;
             }
             else if(userId.startsWith("a"))
             {
                 System.out.println("The ID needs to start with a capital A!");
                 return  false;
             }
-            else if(!userId.startsWith("A"))
+            else
             {
-                System.out.println("The ID needs to start with a capital A!");
-                return false;
+                return true;
             }
-            break;
         case 3: //Professor
-            if(userId.startsWith("P"))
+            if(!userId.startsWith("P"))
             {
-                return true;
+                System.out.println("The ID needs to start with a capital P!");
+                return false;
             }
             else if(userId.startsWith("p"))
             {
                 System.out.println("The ID needs to start with a capital P!");
                 return  false;
             }
-            else if(!userId.startsWith("P"))
+            else
             {
-                System.out.println("The ID needs to start with a capital P!");
-                return false;
+                return true;
             }
-            break;
 
             default: //Error
                 System.out.println("Error");
diff --git a/src/test/java/org/example/AdministrationTest.java b/src/test/java/org/example/AdministrationTest.java
index 048b51b..6d0ca33 100644
--- a/src/test/java/org/example/AdministrationTest.java
+++ b/src/test/java/org/example/AdministrationTest.java
@@ -26,16 +26,16 @@ class AdministrationTest {
 
         Administration administration = new Administration();
         String full01 = "S1002"; //Supposed student id
-        String str1 = full01.substring(1); //String without the Letter
+        // String str1 = full01.substring(1); //String without the Letter
         String full02 = "S100Z"; //Supposed student id
-        String str2 = full02.substring(1); //String without the Letter
+        // String str2 = full02.substring(1); //String without the Letter
         String full03 = "P1004"; //Supposed professor id
-        String str3 = full03.substring(1); //String without the Letter
+        // String str3 = full03.substring(1); //String without the Letter
 
         //checks if the substrings contain only digits
-        boolean test01 = administration.isNumber(str1);
-        boolean test02 = administration.isNumber(str2);
-        boolean test03 = administration.isNumber(str3);
+        boolean test01 = administration.isNumber(full01);
+        boolean test02 = administration.isNumber(full02);
+        boolean test03 = administration.isNumber(full03);
 
         assertEquals(true, test01); //only digits besides the first letter
         assertEquals(false, test02); // false, because the Z at the end