Featuring the FeatureSet

Hi all,

As I’ve discussed before I’m using the python NLTK to predict the Ontology Tags from the Ontology Foundry of microarray annotations. I have a background in machine learning and prediction algorithms but the “natural language” part is new to me. So I’m blindly gathering all of the “features” I can from each sentence and I’ll sort them out later.

The nltk.classify modules require a training set of feature-dict’s and known-labels.  Below is a small snippet of tag/sentence pairs that I’ve removed from the ontologies.  The first term is the ontology tag, the second is the source of the subsequent sentence/fragment: ON represents “official name”, SYN represents “synonyms”, DEF represents “definitions” and COM represents “comments”.

cl:0000052	ON	totipotent stem cell
cl:0000052	DEF	a stem cell from which all cells of the body can form.
cl:0000053	ON	enamel secreting cell
cl:0000050	ON	megakaryocyte erythroid progenitor cell
cl:0000050	DEF	a progenitor cell committed to the megakaryocyte and erythroid lineages.
cl:0000050	SYN	colony forming unit erythroid megakaryocyte
cl:0000050	SYN	cfu-em
cl:0000050	SYN	mep
cl:0000051	ON	common lymphocyte progenitor
cl:0000051	DEF	a progenitor cell committed to the lymphoid lineage.
cl:0000051	SYN	early lymphocyte progenitor
cl:0000051	SYN	committed lymphopoietic stem cell
cl:0000051	SYN	lymphoid stem cell
cl:0000051	SYN	lymphopoietic stem cell
cl:0000051	SYN	clp
cl:0000051	SYN	elp
cl:0000056	ON	myoblast
cl:0000056	DEF	an embryonic (precursor) cell of the myogenic lineage that develops from the mesoderm. they undergo proliferation, migrate to their various sites, and then differentiate into the appropriate form of myocytes.
cl:0000057	ON	fibroblast
cl:0000057	DEF	a connective tissue cell which secretes an extracellular matrix rich in collagen and other macromolecules.
cl:0000054	ON	bone matrix secreting cell
cl:0000055	ON	non-terminally differentiated cell
cl:0000055	DEF	a precursor cell with a limited number of potential fates.
cl:0000055	SYN	blast cell
cl:0000058	ON	chondroblast
cl:0000059	ON	ameloblast
cl:0000059	DEF	a cylindrical epithelial cell in the innermost layer of the enamal organ. their functions include contribution to the development of the dentinoenamel junction by the deposition of a layer of the matrix, thus producing the foundation for the prisms (the structural units of the dental enamal), and production of the matrix for the enamel prisms and interprismatic substance. (from jablonski's dictionary of dentistry, 1992).
fma:36042	ON	proximal epiphyseal plate of fourth metatarsal bone
fma:36043	ON	proximal epiphyseal plate of right fourth metatarsal bone
fma:36046	ON	distal epiphyseal plate of right fourth metatarsal bone
fma:36047	ON	distal epiphyseal plate of left fourth metatarsal bone
fma:36044	ON	proximal epiphyseal plate of left fourth metatarsal bone
fma:36045	ON	distal epiphyseal plate of fourth metatarsal bone
doid:11525	ON	iris and ciliary body vascular disorder
doid:11525	SYN	vascular disorders of iris and ciliary body
doid:11525	SYN	iris and ciliary body vascular disorders nos (disorder)
doid:11525	SYN	vascular disorder of iris and/or ciliary body -retired-
doid:11525	SYN	iris and ciliary body vascular disorder (disorder)
doid:11524	ON	hyphema of iris and ciliary body
doid:11527	ON	laryngostenosis
doid:11527	SYN	stenosis of larynx
doid:11527	SYN	stenosis of larynx (disorder)
doid:11521	ON	hypertensive heart and renal disease with both (congestive) heart failure and renal failure
doid:11521	SYN	hypertensive heart and renal disease with both (congestive) heart failure and renal failure (disorder)
doid:11521	SYN	hypertensive heart and renal disease, unspecified, with heart failure and renal failure
doid:11520	ON	benign hypertensive renal disease
doid:11520	SYN	hypertensive renal disease, benign
doid:11520	SYN	benign hypertensive renal disease (disorder)
doid:11520	SYN	hypertensive renal disease, benign, without mention of renal failure
doid:11523	ON	food poisoning due to clostridium perfringens

Based on the examples I’ve seen in the NLTK documentation I’ve decided to use the presence of certain nouns, adjectives, and noun-adj phrases as my feature sets.  Below is a simple implementation of this:

def LabelFeatures(SENT):

	tokens = nltk.tokenize.word_tokenize(SENT)
	tagged_tokens = nltk.tag.pos_tag(tokens)
	valid_tags = set(['NN', 'NNS', 'JJ'])
	feat_dict = {}
	for this_tok in tagged_tokens:
		if (this_tok[1] == 'NN') | (this_tok[1] == 'NNS'):
			feat_dict['has-noun:(%s)' % this_tok[0]] = True
		if this_tok[1] == 'JJ':
			feat_dict['has-adj:(%s)' % this_tok[0]] = True
	for key_val, group in IT.groupby(tagged_tokens, lambda x: x[1] in valid_tags):
		if key_val:
			g_list = list(group)
			if len(g_list) > 1:
				feat_dict['has-phrase:(%s)' % ' '.join(nltk.tag.untag(g_list))] = True
	return feat_dict

 

 

The decisiontree and naivebayes algorithms will select features (words and word phrases) which occur often in repetitions of the same tag and rarely in different tags.

I’m planning on running a set of simple tests to see how well this will work.  In general, I’ll train on 2/3 of the examples and test on the remaining third.  I also have a small sample (~400 and slowly growing) of hand annotated microarray records that I can use as tests as well.

However, I’m currently running into a MemoryError in the .train methods.  I have a total of 157021 tag/sentence pairs.  I run into a memory error at ~2100 pairs.  I use generators to ensure that I’m not loading the entire file into memory at once, but I think the conditional probabilities in the naivebayes is exploding as I add more training examples.

Any suggestions would be welcome,

-Will

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