This article relies heavily on digital resources, primarily the Excavation Index, a guide to the archaeological excavations and interventions carried out in England since the earliest days of scientific archaeology (Archaeology Data Service 2011). The Index is part of the National Record of the Historic Environment (NRHE) and is currently held online within the Archsearch database of the Archaeology Data Service (ADS), and via a web service through the Heritage Gateway. The dataset used in this study was obtained from the ADS as a set of delimited text files on 9 September 2014. The Index comprises digital information compiled by the Archaeological Investigations Project (AIP) and OASIS, as well as hard-copy information from journals, reports and monographs. As such, the database has historically been the most comprehensive national record for events in the country, and through its wide scope covers much of the shortfall encountered in the individual coverage of AIP and OASIS (Evans 2013; Holbrook and Morton 2008). The significance of the Index as an overarching source of events is perhaps understated in many works that deal with national syntheses and data intensive research. This may be because within the ADS and the Heritage Gateway it can only be searched using associated monument and geographic location, not the classification of the event itself or bibliographic sources. Although the Index can be obtained in its original form from the ADS, its current online incarnation, somewhat ironically, obscures its original purpose.
This is not to say that the Index is definitive. Previous study has shown that the Index contains lacunae, especially when compared to the event records of local Historic Environment Records (Evans 2013). In addition, within the Index there is currently no record for events undertaken after 2011, reflecting its last update within the ADS (Hardman pers. comm.). Furthermore, there seems to be a shortfall in recorded investigations after the years 2007/8, presumably reflecting the ongoing updating of the database via sources such as OASIS and AIP. Despite these caveats, the Index still remains the only consistent national record for investigations. Although it may be assumed that local HER event records provide more information, previous studies have shown that some events and reports are recorded only in AIP or OASIS (and thus the Excavation Index) but not the HER (similarly, some events remain as an ongoing 'backlog' of information and are not yet entered into HERs) (Brookes and Pearce 2003; Evans 2013; Hardman and Evans 2010). Thus, in truth, no one source of events can ever be considered complete; the disparity between systems complicated not only by real gaps, but perhaps also by real or apparent discordance in classification of events (see Evans 2013).
Leaving aside issues of relative accuracy, the rationale in using the Excavation Index for a national study is simply one of practicality. There are currently over 80 HERs in England, a mixture of HBSMR and bespoke systems (MacLean 2014), and although the majority are online via the Heritage Gateway, it is not currently possible to search this system for events or sources. The capacity to obtain and combine the individual datasets lies outside the immediate aims of this work; indeed a comparison of the holdings of local and national systems would make an interesting study for those interested in the epistemological and methodological connotations for the archaeological knowledge base (see Cooper and Green 2015). On this note, the current Historic England Heritage Information Access Strategy (HIAS) looks to increase interoperability between the NRHE and HERs, with enhanced capacity to transfer data to HERs and the enabling of these resources as the first port of call for researchers (see Boldrini et al. 2015). It is hoped that future research into investigation and publication trends will be able to utilise a truly combined system of recording events and outputs, incorporating the strengths of local and national records, and thus allowing a more comprehensive analysis than that currently available. The potential for future developments aside, the Index will continue to play a role for researchers and will be used for this work, although the caveats highlighted above should be noted in any interpretation.
The Library of Unpublished Fieldwork Reports of the ADS comprises the final dataset used for this article and the data cited herein were accessed 22 October 2014 as a download from the ADS. The dataset was restricted to those records that were created through the OASIS system to reflect the current mainstream workflow within archaeology in England. It should be noted, however, that the library contains a large number of reports from other sources, such as backlog scanning projects provided by curators and funders such as Lincolnshire County Council and the Highways Agency. However, these have not been included within the dataset as they represent singular cases of specifically funded projects, not the everyday experience as exemplified by the use, or non-use, of the OASIS system. It should be noted at this point that use of the OASIS system by fieldworkers and curators in England has been uneven (Smith et al. 2012). The data from the ADS Library and OASIS are used here not as a 'complete' dataset but, conversely, as an indicator of the disparities inherent in national patterns of recording.