Another look at employment and market share
A very thoughtful comment was made about HBJ post Employment: Multidisiplinary firms vs. heritage-only firms (18 Feb 2012):
“Large multi-service firms tend hire and terminate for each project because their offices rarely have enough local work to retain technicians. They tend to have centralized full time labs and production centers that do not have positions for techs for after fieldwork tasks. Whereas many hertiage-only company like CRA use full-time technicians in a variety of tasks. The ability to live near a company’s office(s) to come in and do post-fieldwork tasks is the key to full time work. In addition the ability to move techs and other staff between offices reduce the need for temporary project specific techs except for the largest field projects. Plus in any given year we receive enough cold call applicants from technicians with good resumes that simply working the resume file drawer eliminates the need for an ad for most projects.”Steve Creasman and Kay Simpson
A high turnover of technicians in multi-service firms could easily explain why there are more job advertisements from these types of companies than from heritage-only firms. To explore this more, I also took a look at senior positions (e.g. principle investigators, senior archaeologists, office/regional managers) for the same 2011 data set. Of course titles are not standardized across the sector and names can be misleading, but a full 46% of the job advertisements reviewed asked for 10-25 years of experience and 55% asked for 5-9 years of experience (a slight overlap with some asking for 8-12 years). The job descriptions and requirements (years of experience, permits, etc.) firmly place these jobs in the top levels of employment regardless of title. None of these jobs mentioned temporary employment but that does not mean it is not. However, asking for 20 years of experience for a temporary job would be rare but not unheard of (or it should be in my personal opinion). Out of 79 job postings, 61 of them mentioned their employers (some of the job postings have been removed making it impossible to see who was the employer). 
In this data set the breakdown is even more lopsided in favor of multi-service firms
Heritage-only firms may look for their top-level workers through other means than advertisement such as internal promotion or through professional networks. A lack of lower level positions in multi-service firms may make it hard for them to recruit internally. What these data do show is that the majority of job advertisements for archaeologists at all career levels was dominated by multi-service firms in 2011. Does this mean they get the majority of business? That can not be determined from these numbers but employment may indicate strong growth prospects.
Heritage Business Journal 
As a small heritage-only firm, we have never advertised for employees. Like CRA, we get more than enough resumes and cold calls to pick from. Plus we work our network of fellow CRM firms, as well as the local university when we need temporary field people.
Proximity to our office is always a critical factor in terms of whether employment continues past the field phase. Plus, of course, making a good impression as field staff.
When we have made permanent hires, they have often been people who worked as temps for us and impressed us. But we have actually made some very successful hires from resumes or contacts submitted as cold calls by job seekers.
Naturally, like all small firms, the size of our permanent staff is directly dependent on our ability to generate enough work to support full-time people. When we used to be the CRM wing of a multi-disciplinary environmental firm, management was usually able to shift us to other areas when CRM work was slow. As a heritage-only firm, we certainly don’t have that capability.
Out of curiosity, how long did the temporaries have to work with you before a permanent position opened up? It would be interesting to see how long of a time commitment these temps make to the company before a full-time position opens up. It has to be incredibly competitive.