This means the experimenter doesn't know which ants belong to which treatment, so it's impossible to influence the results with 'observer bias'.
1	This	this	PRON	DT	Number=Sing|PronType=Dem	2	nsubj	2:nsubj	Discourse=attribution-negative:131->132:0|Entity=(174-event-giv:act-cf1*-1-coref)
2	means	mean	VERB	VBZ	Mood=Ind|Number=Sing|Person=3|Tense=Pres|VerbForm=Fin	0	root	0:root	_
3	the	the	DET	DT	Definite=Def|PronType=Art	4	det	4:det	Bridge=175<176|Entity=(176-person-acc:inf-cf3-2-sgl
4	experimenter	experimenter	NOUN	NN	Number=Sing	7	nsubj	7:nsubj	Entity=176)
5-6	doesn't	_	_	_	_	_	_	_	_
5	does	do	AUX	VBZ	Mood=Ind|Number=Sing|Person=3|Tense=Pres|VerbForm=Fin	7	aux	7:aux	_
6	n't	not	PART	RB	Polarity=Neg	7	advmod	7:advmod	_
7	know	know	VERB	VB	VerbForm=Inf	2	ccomp	2:ccomp	_
8	which	which	DET	WDT	PronType=Int	9	det	9:det	Discourse=explanation-justify:132->133:0|Entity=(177-animal-new-cf6-2-sgl
9	ants	ant	NOUN	NNS	Number=Plur	10	nsubj	10:nsubj	Entity=177)
10	belong	belong	VERB	VBP	Mood=Ind|Number=Plur|Person=3|Tense=Pres|VerbForm=Fin	7	ccomp	7:ccomp	_
11	to	to	ADP	IN	_	13	case	13:case	_
12	which	which	DET	WDT	PronType=Int	13	det	13:det	Bridge=175<178|Entity=(178-substance-acc:inf-cf5-2-sgl
13	treatment	treatment	NOUN	NN	Number=Sing	10	obl	10:obl:to	Entity=178)|SpaceAfter=No
14	,	,	PUNCT	,	_	18	punct	18:punct	_
15	so	so	ADV	RB	_	18	advmod	18:advmod	Discourse=context-background:133->128:1
16-17	it's	_	_	_	_	_	_	_	_
16	it	it	PRON	PRP	Case=Nom|Gender=Neut|Number=Sing|Person=3|PronType=Prs	18	expl	18:expl	Entity=(179-event-new-cf4-1-cata)
17	's	be	AUX	VBZ	Mood=Ind|Number=Sing|Person=3|Tense=Pres|VerbForm=Fin	18	cop	18:cop	_
18	impossible	impossible	ADJ	JJ	Degree=Pos	2	parataxis	2:parataxis	_
19	to	to	PART	TO	_	20	mark	20:mark	Entity=(179-event-acc:com-cf4-2-coref
20	influence	influence	VERB	VB	VerbForm=Inf	18	csubj	18:csubj	_
21	the	the	DET	DT	Definite=Def|PronType=Art	22	det	22:det	Entity=(171-abstract-giv:inact-cf2-2-coref
22	results	result	NOUN	NNS	Number=Plur	20	obj	20:obj	Entity=171)
23	with	with	ADP	IN	_	26	case	26:case	_
24	'	'	PUNCT	``	_	26	punct	26:punct	SpaceAfter=No|XML=<q>
25	observer	observer	NOUN	NN	Number=Sing	26	compound	26:compound	Entity=(180-abstract-new-cf7-2-sgl(181-person-new-cf8-1-sgl)|XML=<ref>
26	bias	bias	NOUN	NN	Number=Sing	20	obl	20:obl:with	Entity=180)|SpaceAfter=No|XML=</ref>
27	'	'	PUNCT	''	_	26	punct	26:punct	Entity=179)|SpaceAfter=No|XML=</q>
28	.	.	PUNCT	.	_	2	punct	2:punct	_
