  To protect sensitive information in a cross tabulated table, it is a common
practice to suppress some of the cells in the table. This paper investigates
four levels of data security of a two-dimensional table concerning the
effectiveness of this practice. These four levels of data security protect the
information contained in, respectively, individual cells, individual rows and
columns, several rows or columns as a whole, and a table as a whole. The paper
presents efficient algorithms and NP-completeness results for testing and
achieving these four levels of data security. All these complexity results are
obtained by means of fundamental equivalences between the four levels of data
security of a table and four types of connectivity of a graph constructed from
that table.
