protected function source_dropdown() {
		global $wpdb;

		$sources = $wpdb->get_col(
			"SELECT DISTINCT source
			FROM {$wpdb->prefix}woocommerce_log
			WHERE source != ''
			ORDER BY source ASC"
		);

		if ( ! empty( $sources ) ) {
			$selected_source = isset( $_REQUEST['source'] ) ? $_REQUEST['source'] : '';
			?>
				<label for="filter-by-source" class="screen-reader-text"><?php esc_html_e( 'Filter by source', 'woocommerce' ); ?></label>
				<select name="source" id="filter-by-source">
					<option<?php selected( $selected_source, '' ); ?> value=""><?php esc_html_e( 'All sources', 'woocommerce' ); ?></option>
					<?php
					foreach ( $sources as $s ) {
						printf(
							'<option%1$s value="%2$s">%3$s</option>',
							selected( $selected_source, $s, false ),
							esc_attr( $s ),
							esc_html( $s )
						);
					}
					?>
				</select>
			<?php
		}
	}