During the past 18 months the Irish economy has experienced a flow of immigration the magnitude and stability of which are difficult to establish. So also is the impact of this inflow on our national output.
In the March-May period of 2004, our Quarterly National Household Survey showed there were 29,000 people from eastern Europe in our labour force, half of them Poles.
We don't know how many dependants were accompanying them, but, including dependants, the total number of people here from east Europe could have been about 50,000.
In the March-May period of this year, 27,000 people living here had 12 months earlier been "usually resident" in eastern Europe, just under half of them Poles.
If that accurately reflected immigration from these countries in the 12 months ended March-May 2005, then this fresh inflow could have brought the total number of east European nationals living here up to something around 75,000.
And about two-thirds of this 75,000 would have been in our labour force.
We also know that between May 2004 and April 2005 a total of 85,000 people from eastern Europe - once again just under half being Poles - registered for employment here.
There are indications that quite a high proportion of such workers only lived here temporarily, returning home with their savings after a limited stay.
Thus this higher registration figure may be compatible with a figure of 50,000 eastern Europeans in our labour force at the present time.
The very large change in the scale and flows of immigration since May 2004 may go some way - although not the whole way - towards explaining the uncertainty that now exists with respect to our recent economic growth.
Since the first estimates of economic growth in the years 2002 and 2003 were made by the CSO, that office has found it necessary to increase both these growth estimates by about 2½ percentage points, and it is clearly possible that similar upward adjustments may eventually have to be made both to the CSO's initial estimate of 4 per cent growth of GNP in 2004 and to its estimate of a 3.3 per cent growth rate in the first half of the current year.
That is, in fact, the belief of the Economic and Social Research Institute (ESRI), which holds the view that in the first half of the current year growth was in fact a good deal higher than the 3.3 per cent estimated by the CSO.
For this year as a whole the ESRI has recently raised its forecast from a growth rate slightly below 5.5 per cent to a figure slightly above that figure.
In August the Department of Finance also slightly increased its GNP forecast for the current year from 4.7 per cent to 5 per cent.
Yet this week the Central Bank did the opposite and reduced its output forecast for the current year from 5.25 per cent to 4.5 per cent. This may prove nearer the mark than the ESRI's 5.6 per cent.
The ESRI has defended its raised growth projection by reference to the fact that in the 12 months ended March-May last employment grew by a quite extraordinary 5.1 per cent.
If the CSO's estimated growth rate of 3.1 per cent for that period was correct this would mean that output per worker (labour productivity) would have actually declined by 2 per cent over that 12-month period.
The ESRI sees this as so improbable that it believes the CSO output figures must be too low.
It may, however, be relying too much on these employment data.
If, however, the ESRI is right, one must wonder what form the missing output took.
Unfortunately the ESRI does not offer any guidance on this point: it publishes figures only in respect of the way in which we dispose of our output: by consuming it, investing it or exporting it.
The only clue the institute has offered about actual output trends is its estimate of a €4.5 billion rise in the exports of goods in the current year.
However, we now know that there was no increase in the volume of such exports during the first eight months of this year.
This means that for the ESRI export target for the year to be achieved it would be necessary for our stagnant export trade to be galvanised into a growth rate of over 10 per cent in the last four months of the year. This seems most improbable.
Therefore if the ESRI is right about a high growth rate this year this must have derived from a much higher level of activity in construction and market services than the CSO has hitherto detected.
It is possible, for example, that there has been an undetected increase in house repairs by eastern Europeans coming here after EU enlargement in May last year.
Some proportion of the extraordinary 37,000 (18 per cent) increase that has taken place in the number of workers in construction in the 12 months that ended in the March-May period must surely have been accounted for by that inflow of eastern European workers, many of them working here at low rates of pay.
During the period since May last year the availability of cheap labour from eastern European countries may have contributed to a perceptible once-off shift in the structure of our labour force. Many employers have clearly been taking advantage - in some cases exploitative advantage - of this new source of cheap labour.
This, however, has made it very difficult to measure output and productivity during this recent period of exceptional change in our labour market.
Of the extraordinary increase of 93,000 in our workforce during the 12 months to April last, net inward migration accounted for 36,000, and changes in the age structure of our population explained a further 18,000.
The remaining 39,000 extra workers reflect a further rise in the participation rate, ie the proportion of people aged between 15 and 64 who are at work.
This has been due to a further significant rise in the participation rate of women, especially married women over 45, combined with an apparent rise of almost 10 per cent in the proportion of people aged 60-69 who are remaining at work.
Finally, it is particularly significant that, despite large-scale immigration, the unemployment rate actually fell slightly during this 12-month period.
It will clearly take some time for the uncertainties created by the current conflicting and confusing economic indicators to be sorted out.