The rules on whether Ontario cabinet ministers who slip up can be forced to resign are almost as elastic as Spandex athletic pants and often can be shaped to whatever best fits a government.

The opposition parties are finding this out after calling for weeks for the firing of David Caplan, the minister responsible for lotteries, whose ministry failed to get to grips quickly enough with the huge issue of ticket retailers defrauding ticket buyers.

Precedents show ministers under fire are more likely to resign when their failings fall within easily defined, clear-cut categories. The most common in recent years has been making public information that broke privacy laws.

Under former premier Mike Harris, Bob Runciman was solicitor-general when his ministry contributed information toward a throne speech that identified a young offender and, although he did not divulge the information personally, he briefly stepped down.

One of health minister Jim Wilson's aides told a reporter a doctor who had criticized government policies on billings was a particularly high biller, although divulging individual doctors' billings is forbidden. Everyone involved said Wilson never knew about it, but he similarly left his job for a while.

In the preceding New Democrat government, health minister Evelyn Gigantes blurted out under questioning the identity of a drug addict and was quickly gone, but another minister, Shelley Martel, who mentioned having seen incriminating evidence about a doctor in his confidential file, changed her story and the government ignored many demands she be fired.

Ministers clearly have to go when they are charged with offences and Will Ferguson resigned as NDP energy minister after police charged him with helping a teenager escape from a girls' training school, while he was a student social worker, in return for sex. A court cleared him, but he never got back in cabinet.

Ken Keyes, a solicitor-general under Liberal premier David Peterson, was cast adrift for taking foreign guests on a police launch where they were served alcohol, when the law allowed drinking only in residences or licensed premises.

Greg Sorbara, the current Liberal finance minister, had to step down merely because police were investigating a company with which he had been involved - the wealthy, entrepreneurial Sorbara has been involved with more companies than Conrad Black - but he returned as finance minister after a judge said his integrity should never have been questioned.

Ministers have also been dropped for interfering or seeming to interfere with legal processes. Conservative solicitor-general George Kerr had to resign because he phoned an assistant crown attorney hinting that a constituent convicted of a driving charge should not be sent to jail.

A worker in the constituency office of NDP solicitor general Mike Farnan wrote on his letterhead to a justice of the peace complaining that a resident was given a parking ticket by mistake and, although Farnan had not authorized the letter, he had to take responsibility and resign.

Opponents have argued that Caplan should follow the same course as his mother, Elinor, who resigned as chairman of management board of cabinet in the 1980s, when it was revealed her husband was a business consultant for a company that obtained financing from the province.

She said she was unaware of husband's involvement with the company and was respected enough that many believed her, but it put her in a conflict of interest.

David Caplan is not accused of having a conflict, which often can be clear-cut, but rather of failing to do his job adequately, which is more open to interpretation.

Whether a minister being criticized resigns also depends a lot on how much a premier is prepared to support him, and Premier Dalton McGuinty has clung to troubled ministers more than most premiers.

McGuinty refused to drop Harinder Takhar, who failed to sever himself fully from his former business, despite being attacked on it almost daily for a year, and seems determined to hang tough again. And his opponents have few weapons to convince him otherwise.