In 1968, when America despaired over the state of its inner cities and the Kerner Commission foresaw the evolution of "two societies, one black, one white -- separate and unequal," the official unemployment rate among American blacks was eight percent.

In 1980, when the memory of urban rioting had begun to fade until the uprising in Miami suddenly revived it, the official unemployment rate among American blacks was about 12 percent, or 50 percent higher.

There have been vast changes and many improvements in the status of some black Americans since 1968, but in most measurable ways the conditions in which the poorest black Americans live have failed to improve. In many respects they have deteriorated.

Whether this means more riots like Miami's are in prospect remains unknowable. Experts are divided, and the evidence is inconclusive. If nothing else, the Miami riot seemed to demonstrate that the illogic of risking one's life to destroy one's own neighborhood is not enough to dissuade people from choosing to riot under some circumstances. What is not in doubt is the fact that the circumstances of life in American's ghettos remain grim.

In fact, those official unemployment statistics dramatically understate the real level of joblessness among blacks, according to studies by the National Urban League calculates that the real unemployment rate among blacks in the first quarter of 1980 was 22.8 percent, counting people who long ago gave up looking for a job and others not counted in the official Labor Department computations.

But even that huge figure understates part of the problem. For the youngest would-be members of the work force, teenagers, the actual unemployment rate for blacks is around fifty percent. It has gotten much worse since the riots of the 1960s.

There is a little good news about the evolution of ghetto life since the last time mainstream America was compelled to pay attentin to it. The quality of ghetto housing is apparently improving, at least as measured by such considerations as the presence of plumbing and the degree of overcrowding. Some of the worst inner-city housing has disappeared since the late 1960s, some because of arson, some because of landlord abandonment, some because of urban renewal.

The decline in overcrowding reflects another positive trend; the population in the impoverished inner cities is declining, in part because of a declining birthrate. The number of young blacks between 18 and 24 grew from 2.8 million to 3.8 million from 1970, but that number will decline slightly during the 1980s.

And there is a lot of good news about expanded opportunities for blacks in general. Leaders of the inner-city poor in the 1960s -- Washington's mayor, Marion Barry, for example -- have become leaders of their communities.

In 1969 there were 1,185 elected black officials in the United States. Last year the number was 4,607, a 288 percent increase. Blacks have been able to move into suburbs (though usually segregated ones), to attend college in much greater numbers, to enter careers previously all but shut off to them and more.

But the principal beneficiaries of those changes have been middle-class people. The all-but-forgotten under class has much less to show for the 1970s. When the '70s began about seven million blacks lived below the government's official poverty line. Today there are nearly eight million blacks in that category.

Poor inner-city blacks suffer disproportionately from crime, and the incidence of reported crime has soared since the late 1960s. In 1970, there were 360 cases of murder, robbery, rape and assault per 100,000 of population. In 1977 that figure was up to 460. The changes since 1960 have been more dramatic. That year someone was robbed, on the average, every six minutes. By 1977 there was a robbery once every 78 seconds.

The schools in the urban ghettos continue to turn out semi-literate or illiterate students. Jobs have continued to move from cities to suburbs, as has the middle class of both races.

Tensions between ghetto residents and the police have been a traditional source of conflict, sometimes leading to riot, as in Miami last weekend. In some big cities black mayors and/or black police chiefs have made substantial efforts to improve police relations with poor blacks, often by hiring more black police. But the proportion of black police officers in big cities remains low, with a few dramatic exceptions like Washington (50 percent) and Detroit (more than 60 percent).

According to a recent Associated Press survey, the percentage of blacks on the Philadelphia police force is 19; in Memphis it's 18; in Baltimore 17; in New York 8; in Los Angeles 6. Some cities have established elaborate citizens' review procedures and other measures to try to ease police relations with minority communities, but many other cities have done nothing. (Miami had recently set up a citizens' review board; this was no guarantee of tranquility.)

The connection between bad living conditions, tense police-community relations and riots is tentative at best. Those factors are almost always present when a riot erupts, but riots have never occurred in many other places where those circumstances also exist.

Prof. Louis Masotti, director of the Center for Urban Affairs at Northwestern University, studied riots for many years. He said in an interview that he thought that one big riot tended to serve as a model for angry citizens in other cities, particularly those close by. Small cities around Detroit and Newark both suffered from a rash of civil disorders after the big riots in those cities, he observed.

Masotti said he thought conditions would be "right" for riots in other cities this summer, and that Miami gave potential rioters a model for their behavior, but he declined to make any predictions.

He said he saw a stronger tendency now toward organized criminal activities by gangs of black and other minority youths operating out of ghetto neighborhoods. Gang criminality -- selling protection and the like -- represents "economically rewarding protest," whereas riots are more "psychologically rewarding," Masotti said.

John McConnahay, a professor at Duke University who studied the participants in the Watts riot of 1965, theorized that rioting now comes to America in 20-year cycles that began in the early 1940s. "The next riots should have come in 1984," McConnahay said, "but everything's happening faster now."