diff --git a/lib/src/data_frame/series.dart b/lib/src/data_frame/series.dart
index b572c9e..f2784e9 100644
--- a/lib/src/data_frame/series.dart
+++ b/lib/src/data_frame/series.dart
@@ -1,6 +1,13 @@
 class Series {
-  Series(this.name, this.data);
+  Series(this.name, this.data, {
+    bool isDiscrete = false,
+  }) : discreteValues = isDiscrete
+      ? Set<dynamic>.from(data)
+      : const <dynamic>[];
 
   final String name;
   final Iterable data;
+  final Iterable discreteValues;
+
+  bool get isDiscrete => discreteValues.isNotEmpty;
 }
diff --git a/test/data_frame/data_frame_test.dart b/test/data_frame/data_frame_test.dart
index 65ab113..a880d5d 100644
--- a/test/data_frame/data_frame_test.dart
+++ b/test/data_frame/data_frame_test.dart
@@ -281,5 +281,37 @@ void main() {
       expect(frame[2].name, 'third');
       expect(frame[2].data, equals([3, 323, 1000]));
     });
+
+    test('should return null if one tries to access a series using a key of '
+        'improper type (neither String nor int)', () {
+      final data = [
+        ['first',  'second', 'third'],
+        [  '1',        2,         3 ],
+        [   10,       12,       323 ],
+        [  -10,      202,      1000 ],
+      ];
+      final frame = DataFrame(data, headerExists: true,
+          columnNames: ['col_1', 'col_3', 'col_4']);
+
+      expect(frame[{1}], isNull);
+      expect(frame[1.2], isNull);
+      expect(frame[[1, 2]], isNull);
+    });
+
+    test('should throw a range error if one tries to access a series using an '
+        'integer key which is out of range', () {
+      final data = [
+        ['first',  'second', 'third'],
+        [  '1',        2,         3 ],
+        [   10,       12,       323 ],
+        [  -10,      202,      1000 ],
+      ];
+      final frame = DataFrame(data, headerExists: true,
+          columnNames: ['col_1', 'col_3', 'col_4']);
+
+      expect(() => frame[3], throwsRangeError);
+      expect(() => frame[4], throwsRangeError);
+      expect(() => frame[-1], throwsRangeError);
+    });
   });
 }
diff --git a/test/data_frame/series_test.dart b/test/data_frame/series_test.dart
new file mode 100644
index 0000000..b9cf47d
--- /dev/null
+++ b/test/data_frame/series_test.dart
@@ -0,0 +1,26 @@
+import 'package:ml_dataframe/ml_dataframe.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group('Series', () {
+    test('should initialize properly', () {
+      final data = [1, 2, 3, '4'];
+      final series = Series('series_name', data);
+
+      expect(series.name, 'series_name');
+      expect(series.data, equals(data));
+      expect(series.isDiscrete, isFalse);
+      expect(series.discreteValues, isEmpty);
+    });
+
+    test('should initialize as a series with discrete data sequence', () {
+      final data = [1, 2, 3, '4', 1, 2, 10, '4'];
+      final series = Series('series_name', data, isDiscrete: true);
+
+      expect(series.name, 'series_name');
+      expect(series.data, equals(data));
+      expect(series.isDiscrete, isTrue);
+      expect(series.discreteValues, equals([1, 2, 3, '4', 10]));
+    });
+  });
+}
diff --git a/test/numerical_converter/numerical_converter_impl_test.dart b/test/numerical_converter/numerical_converter_impl_test.dart
new file mode 100644
index 0000000..44eab59
--- /dev/null
+++ b/test/numerical_converter/numerical_converter_impl_test.dart
@@ -0,0 +1,7 @@
+import 'package:test/test.dart';
+
+void main() {
+  group('', () {
+
+  });
+}